Question for the baker

First I was wondering how much should I expect my cake to come to?

I have 260 on the guest list but expect 160.
I was thinking a small cake to cut and for my topped and then mini cakes for each guest.

I was also thinking of having a small grooms cake.

Second when should I go in to order it? Florist have already been booked up for my wedding and that was 4 months ago!

Thanks