I've turned exactly one plate before, but I'm now preparing to turn platters for my brother and sister to commemorate and celebrate our dad who passed away last year. I'm doing a bit of research to solidify my design ideas and I thought I would ping the hundreds of years of collective experience here. I'm thinking a wide brim platter, but what proportion is the brim and foot?
The real answer is "whatever proportions you like"

but is sure is easier to get some ideas before you decide! Some people insist on "rules" for various proportions but IMO that's hogwash - I ignore those and make things so they look right to me.
I've done a few platters, here is a pair made of sapele (sometimes called sapele mahogany):
The larger one is 18.5" with an 8" wide foot.
The smaller one is 14" with the foot about 6".
Since I made them as a pair for my wife, I decided to make the brims the same - they are about 2-1/4" wide.
The tops are about 1-3/4" from the table - made from 2" slabs.
I think this platter is about 19", also made of sapele.
I don't know the foot or rim size since someone else has it but it would be easy to measure from the picture.
For something completely different, I've made a number of these for gifts and they are always well liked. Some are flatter on top and a little shorter and might be called platters and some are more deeply dished and might be called shallow bowls. I make them about 10" at the widest points, the foot is just over 4" wide, and the top is about 1.5" from the table. (made from 2" stock)
JKJ