I have a wedding coming up on July 22. 

When I first received the order/ did the tasting / took deposit t (May 12) there was a guest list of 65, this is what we designed the cake for and I charged accordingly. About every two weeks since the number of guest has increased (78, then 90, now we are at 100 confirmed today).  Of course I'm increasing the price, have had to change the stand and increased the delivery fee now that I need an assistant, minor inconveniences, but I'm wondering if anyone else has encountered this? I'm taking their final payment on Wednesday and wondering if I should include a final cake size that I wont budge on? I don't want to be rude but I need this order finalized. 

Second question
The couple is now telling me they don't plan on having a caterer cut the cake. I've only done a few weddings and have no experience with this. Should I suggest a larger cake than normal? Caterers and venues are generally very careful to cut "Wedding cake slices" I just cant imagine these people getting 100 proper and properly sized slices out of their cake and dont want to come up short. 

I think it might be a good idea to include a final cake size.  If she simply HAD to have a few more servings, you could always add cupcakes or a cutting (sheet) cake.

Explain to your client that "servings" must be a specific size and without an exerienced person to cut the cake, you recommend going with a larger cake.  If she chooses to stay with the same size cake, provide a serving chart when you deliver the cake.

I allow serving increases until the final payment date which is 30 days before the event. Changes after that are charged a change fee of $50 per change. Usually just putting a change fee on something is enough to "encourage" them not to go past the final payment date with changes! :)

Yes, you should insist on no more changes - or give them a date like 2 or 3 weeks before.  After whatever date you decide on there can be NO changes. 

As for serving the cake - are they asking you do do that? Charge a fee!  It's your time away from family &/or business.   OR are they going to have some family member/friend do it?   Just give them a drawing of how to cut it.  It's up to them to make it go as far as it's suppose to.    I remember one time when it was a friend's wedding - I made the cake but her daughter was to cut it.   She was a BIG gal and I could not believe the size of cake she was going to serve - pieces about 4X the size!  I stepped in and offered to serve :) 

I include a cutting chart with every cake.