If you are having a large wedding with 100+ guests then you need to ensure that you book a venue that can accommodate a large number of guests. Once you have your venue booked check with the venue regarding table layouts.
They may have a standard layout or will let you move the tables around to suit your needs. You can use our drag and drop seating planner http://www.seatingplanner.co.uk to layout the tables as they would be at the venue and then seat the guests where you would like them.
With a large wedding you will most likely have some children at the reception. Its a good idea to put some space aside to provide some activities whilst the speeches are underway. Maybe to seat the children on a separate table and place some colouring books or games to keep the children occupied.