I will give you my two cents, I am sure others will be along to give theirs as well
My current (and first) table was 20" by 30" which has served me well over the years. Then I bought a 24" Incra Jig...the table is seriously undersized for this and I am planning to create a new top and eventually a cabinet to set it on. The new dimensions will be on the largish size of 28" by 42". This serves several purposes. It lets me take advantage of the long Incra Jig (how far back do you anticipate the fence going?), gives me room in front of the fence for working with large pieces (cabinet doors and the like), and I plan to use the table for an assembly area (cramped garage space). So short answer is, it really depends on your needs and what you plan to do on the table.
Since I will be using the table for assembly as well, I was planning on about a 2" overhang. There seems to be an endless need to clamp things to the table and the overhang will provide enough depth to get the job done.
If you are going to route out the insert opening, you should do this with the laminate on, otherwise you will not correctly account for the thickness of the laminate. There are a few different ways to route the opening that I have seen. For me, I plan to make a template out of scrap, test that with the real plate (maybe even do a test route on scrap), and use a pattern bit to cut the shoulder, then cut out the remainder with a jig saw. The upside with the template is that if it doesn't come out quite right you can start over without having ruined your table top (like I did with my first table
I have just downloaded the free drawing tool, SketchUp from Google and plan to make a drawing of this over the next week. I will post that when I get it done, as I am also interested in the groups feedback.