New at this ...

If I have many different sizes of empty gem on me and I capture a soul, will the soul go to

( a ) the smallest gem it will fit in? Or
( b ) a random gem?

A)

there is a mod that makes sure souls only to the proper soul gem so you dont have petty souls in your grand gems.

I am avoiding using mods at the moment.

So there is a chance that a petty soul will go to larger soul gems even if I have empty petty gems?

Souls go to the soul gem of the corresponding size as long as you have an empty one on you. If you kill a skeever but dont have a petty soul gem, it will go to the next smallest. If your next smallest gem is a greater soul gem, it will fill it. You'll want to note that SOUL GEMS FILLED WITH THE WRONG SIZE SOUL CAN BE EMPTIED SIMPLY BY DROPPING THEM ON THE GROUND AND PICKING THEM BACK UP.

sorry for the caps, its just an important thing to know. and no, there is no chance for a petty soul to go to a larger soul gem as long as you have an empty petty gem

Thank you both. Good to know.

Thankyou, that's cleared some things up for me, too.

Yeah... Good to know that. Thanks for this information. I hate having a great gem filled with a petty soul...