When ordering it says seat size 16 or 17 & then seat I.D.

It is basically the OD of the reel seat.  17 has a larger profile than a 16.

if you are going to order a casting reel seat , i advise you to go with a size 17

For spinning rods I really like a 17 over a 16, the larger O.D. is significantly better for ergonomics than a 16, more comfortable on a long day of fishing.  As the size goes up the weight goes up, but not appreciatively on 16-17..  Also, you may find the length is longer  on the larger sizes (seats are available in 18's, 19's, and at least 20's, too, but I think most would consider 18 about as big as one would normally go on a fresh water rod.  The longer length is usually unnecessary so many builders cut the threads off on even 16's.  Keep in mind your current reel may not have the longest foot you might want to try in the future.

 

I would appreciate someone explaining the differences between the 16 and 17 casting seat.  The bore at the rear is larger on a 17 requiring non-"standard" cork and fittings?  thanks

This is going to be a casting rod, had to go with the 16 for the 14 mm for the exposed rod seat for rod diameter, the reel seat is a Fuji ECSM. The 17 started at 15mm

Where will the 14 mm fit? You'll want 8-11 behind the seat. I'm average build and find that 10" grip on a 7' bass rod is long enough for two hand casting but won't get hung in clothes.

All of my personal rods are built with size 16 reel seats.  You will like it.

Around 10" - 11"

wait wait wait.. First off.. I want to interject that size matters to only the person using the seat. Some like certain sizes some don't. 

 

Ok now.. size 16 and 17 does refer to "Size" of the reel seat. But remember some real seats also have ID sizes that are smaller. Thus, if you like a larger reel seat but want to fit it on a smaller diameter blank, you can. Our seats different in sizes per model. Its best to feel whats comfy to you then decide.