Recent discussions have highlighted the importance of texts in which a variation of the term "appear" or "appearances" (as translated in English) are to Epicurean Canonics. For example, in the size of the sun issue, the Epicurean position seems to distill down into something like: "The size of the sun is as it appears to be."
For we must not conduct scientific investigation by means of empty assumptions and arbitrary principles, but follow the lead of phenomena: for our life has not now any place for irrational belief and groundless imaginings, but we must live free from trouble. Now all goes on without disturbance as far as regards each of those things which may be explained in several ways so as to harmonize with what we perceive, when one admits, as we are bound to do, probable theories about them. But when one accepts one theory and rejects another which harmonizes as well with the phenomenon, it is obvious that he altogether leaves the path of scientific inquiry and has recourse to myth. Now we can obtain indications of what happens above from some of the phenomena on earth: for we can observe how they come to pass, though we cannot observe the phenomena in the sky: for they may be produced in several ways. Yet we must never desert the appearance of each of these phenomena, and further, as regards what is associated with it, we must distinguish those things whose production in several ways is not contradicted by phenomena on earth." - Letter to Pythocles 
As to the Sun itself in the letter to Pythocles:
 The size of sun (and moon) and the other stars is for us what it appears to be; and in reality it is either (slightly) greater than what we see or slightly less or the same size: for so too fires on earth when looked at from a distance seem to the senses. And every objection at this point will easily be dissipated, if we pay attention to the clear vision, as I show in my books about nature.
The issue is discussed by Lucretius this way in his Book Five (Bailey):
 Nor can the sun’s blazing wheel be much greater or less, than it is seen to be by our senses. For from whatsoever distances fires can throw us their light and breathe their warm heat upon our limbs, they lose nothing of the body of their flames because of the interspaces, their fire is no whit shrunken to the sight. Even so, since the heat of the sun and the light he sheds, arrive at our senses and cheer the spots on which they fall, the form and bulk of the sun as well must needs be seen truly from earth, so that you could alter it almost nothing to greater or less.  The moon, too, whether she illumines places with a borrowed light as she moves along, or throws out her own rays from her own body, however that may be, moves on with a shape no whit greater than seems that shape, with which we perceive her with our eyes. For all things which we behold far sundered from us through much air, are seen to grow confused in shape, ere their outline is lessened. Wherefore it must needs be that the moon, inasmuch as she shows a clear-marked shape and an outline well defined, is seen by us from earth in the heights, just as she is, clear-cut all along her outer edges, and just the size she is.  Lastly, all the fires of heaven that you see from earth; inasmuch as all fires that we see on earth, so long as their twinkling light is clear, so long as their blaze is perceived, are seen to change their size only in some very small degree from time to time to greater or less, the further they are away: so we may know that the heavenly fires can only be a very minute degree smaller or larger by a little tiny piece.
It would probably be helpful to look into the number of locations where that kind of formulation occurs in the text, and see what Greek (and Latin) words were used in the originals.
Since I am more familiar with the Latin, here is a section from Lucretius:
Nec nimio solis maior rota nec minor ardor
esse potest, nostris quam sensibus esse videtur.
nam quibus e spatiis cumque ignes lumina possunt
adiicere et calidum membris adflare vaporem,
nil magnis intervallis de corpore libant
flammarum, nihil ad speciem est contractior ignis.
proinde, calor quoniam solis lumenque profusum
perveniunt nostros ad sensus et loca fulgent,
forma quoque hinc solis debet filumque videri,
nil adeo ut possis plus aut minus addere vere.
The Loeb / Rouse / Smith edition translates that as:
The wheel of the sun and its heat cannot be much greater or less than is perceived by our senses.
The word "Appearance" can have many shades of meaning in English. Several of the major meanings carry negative connotations in English, as if we should presume that anyone who uses the word "appears" is actually "mistaken" or even "deceived." Merriam-Webster:
Definition of appearance
1a : external show : semblance Although hostile, he preserved an appearance of neutrality. b : outward aspect : look had a fierce appearance c appearances plural : outward indication trying to keep up appearances 2a : a sense impression or aspect of a thing The blue of distant hills is only an appearance. b : the world of sensible phenomena 3a : the act, action, or process of appearing the first appearance of that word in English b law : the presentation of oneself in court as a party to an action often through the representation of an attorney 4a : something that appears : phenomenon b : an instance of appearing : occurrence her first public appearance since winning the award
I doubt strongly that Epicurus intended to attach a "negative" implication to these constructions about the size of the sun. In other words, I don't think he meant to imply that his own formulations were mistaken or deceptive. That would be a subset of the entire question of how to regard the senses and their reliability. I think we need to find the best ways to state this issue in both far and firm terms, so that people can understand what Epicurean philosophy expects from the senses, and what is beyond the limit of their capability.
So I am starting this thread as a placeholder for this conversation, because I think if we look at the instances in the texts where references like this occur, we can get a better understanding of the degree of firmness which Epicurus is attaching to these statements.
When we see in English that the word "Appearance" is being used, should we substitute (at least in our minds) a form of the word "Perception"?
Such as "The size of the sun is that which we perceive it to be." (?)