sizes of objects used for Alchemical Charms

[Varyon]

If you want to go this route, I can see three variables that can come into play - volume (the item's SM), weight, and cost. Large items are less convenient to carry around, heavier objects use up more of your carrying capacity, and of course more expensive objects cost more, so as each of these go up, some sort of bonus comes into play (and as they go down, there's some sort of penalty). You'd need to come up with what you want the default to be for each on your own. You could also have certain materials function as though they were larger/smaller, heavier/lighter, and/or more or less expensive than they really are for purposes of magic - perhaps gold is really conducive to magic while iron is rather hostile to it, so a gold ring functions as though it were worth twice as much as it really is and an iron sword functions as though it were worth half as much as it really is.

As for the bonus/penalty, I see two obvious options. The first, as you note, would simply be a bonus/penalty to the relevant skill. The other would be a reduction/increase to the amount of time required to make the charm.

As for scaling, my inclination would be that every +1 to SM, +3 SSR to weight, and +3 SSR to cost, are altogether worth either a +1 to skill or perhaps a -1 SSR to the amount of time required. Note that's having all three of those be the case - each of those alone is worth only +1/3 to skill or -1/3 SSR (~x0.9) to time. You'll have to decide for yourself what a +0 item's size/weight/cost would be.