is there a rule of thumb for deciding the size of the grooms cake? ex: if the wedding is planned for 300, do you plan the grooms cake to feed 300?

If the grooms cake was to feed 300 and they are planning on 300 people being there I'm assuming there isn't a wedding cake???
If the grooms cake needs to be big for servings then make it big, if not then don't. It would also depend on the idea/theme.
I guess the question would be "how many people does the grooms cake need to feed and then does the theme/idea support that?"

There is a wedding cake, and the reason for the question is that i don't know if it needs to be large or small with a plan of 300 people. It is a hunting/fishing theme. I'll change the question: how many people should a grooms cake feed?

I understand now. If the wedding cake will feed 300 then the grooms cake can be as big as they/you want. If the size doesn't matter at all I would make it appropriate to the design. Like, if it's going to be a fish make it life sized so it's more realistic. Or, ask them how large they would like it to be. They might have a specific idea in their head about the size that you just don't know yet?
I saw just the other day on TV a grooms cake that would have fed at least twice the people as the wedding cake (Amazing Cakes). So in their wedding the grooms cake was more important than the actual wedding cake.
Are you making the wedding cake too?

I'm actually making it for my sons wedding, so its kinda in my hands to decide it how ever I want it. NO! I'm NOT making the wedding cake!! (This venture is enough!) Thanks for the help.

HAHAHA, good for you!!!! (on NOT doing the wedding cake)
Since your the "mother of the groom" I would make the cake exactly how YOU want it!!
If your making a fish I can give you some pointers on the one I did. ?? It was a lot easier than I thought it would be!!
Good luck and (pre) congrat's on your Son's marriage!

Rule of thumb is that you make it in addition to the wedding cake servings so that both make up the 300 servings. Well, that's in a perfect world. In my experience there are always lots of people who want to try BOTH cakes. SO if I needed 300 servings I would round it up to about 325 and then make the grooms like 50 and the brides cake 275. That way everyone can get at least one piece of cake and those who want to try both can try both.

See if you can find out how many servings are in the brides cake. If they have already ordered 300 servings of that cake then you could literally make the grooms cake any size you want. I'd average about 30 pieces.

Cat