francofs

It's not at all that easy. I have encountered creatures with big souls that are easy to kill and others that have small souls that were not that easy to kill. And it's specially difficult to determine between two close soul sizes. For example, if you are out of common soul gems, having only lesser and greater, failing to determine between common and lesser would waste a greater soul gem. That's a nice idea.

While I agree it's a little immersion breaking, but having no clue is worse. First because currently, if you have lots of empty soulgems in the inventory it's hard to know which soulgem the creature filled, so in many situations you can't get to learn creature's soul sizes. Second it's really a pain having to memorize what all monsters are not worth the immersion, specially for those that have difficulty in memorizing stuff like this, like me. A good sould gem mod would allow to configure the level of aid on this and everyone could decide how much of immersion breaking the want to have. If they want none, just don't use the mod. The way it is right now is a real pain for enchanters :(

Is there a mod that displays the target's soul size or prevent/askToConfirm smaller souls filling bigger soul gems?

It's a pain not to be sure what size of a soul a target has. It would be very useful to know that to avoid having small souls filling big soul gems. The mod could be configured to display the soul size of the target all the time or in certain situations only like when having a soul trap spell ready. Additionally there could be a confirmation box to wether or not absorb a soul if it's smaller than the smallest available soul gem.