I have always used size 16 seats as a most all my rods are freshwater light to mag bass. I'm building some heavier 10 to 13 foot spinning float rods currently and I'm wondering if I should use size 17 seats. I really don't like the feel of them but I can see reel size of 4000 or so. I personally still like 3000-3500 series but you just never know.

17 or even 18. I don't use 16's on anything any more, just too small, makes my hands hurt when used for a long time.

From a pure ergonomics standpoint, anything smaller than about a 22 would be considered incorrect for all but he tiniest human hands. However, if you are able to be comfortable with something smaller then go that route. Anything above a 16 will typically hold about any reel foot you're likely to run into.

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For years, I avoided spinning rods and spent a great deal of time and effort finding rods and reels that could handle some of the smaller finesse lures that I wanted to use for bass. Once I started building and finally got around to building some spinning rods with rear grip lengths and reel mounts that suited me ergonomically, I found the spinning rod way more useful.

For me, 18mm is the sweet spot if I need to use a reel seat on a freshwater spinning rod. I prefer to wrap my spinning reels on with thread on a section of woven graphite just big enough to fill my hand. The OD of the grips are 1.05", which is pretty similar to the OD of an 18mm seat. The only spinning rod I still have a seat on is a little light power travel rod. It has an 18mm seat and is paired with a little size 10 Quantum reel. I have pretty small hands and have found that this is the smallest I can go on a bass rod that won't send pain shooting up my forearm on a hookset, something that started in my early 20s.

If the smaller seats suit you well and feel comfortable, by all means run with it.

All my older G Loomis, Lamiglas, St Croix factory rods, even the ultralights had at least a 17 mm seat, my son's cheap med light Cebelas travel rod has an 18 mm Fuji DPSM. I use a lot of 20 mm NPS seats, 18 mm on ultralights and lights as a minimum.

I only use 16's on light rods. !7 is only a little beefier (in my opinion) in looks and feel. I agree with other commenters on even going 18 or larger. Maybe more balance.

I size my reel seats to fit my hand, not the rod. All my rods have no less than a size 20, including ultra-lights.

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Could always try a Fuji VSS 17 or IPS 16 reel seat with a hidden thread hood with sleeve. The OD of the finished cork reel seat will be about 27 mm OD, very comfortable to hold.
Norm