Depends on if it's a standard narrow spool, wide spool, or even a large arbor. I believe for old Hardy reels the sizes in 1/8" increments are the trout reel sizes with narrow spools, and the reel sizes in 1/4" increments are the wide spool salmon reels.
Some classic reel forums have scanned old reel papers that give the capacities for various classic reels.
Wait until you get into Spey rod fishing. Your reels come in Oh my god sizes. I had a 14' St Croix which I had a reel about 6" in dia. It held a 9,10,11 line. My leaders were 15'. All hand tied. It was no fun trying to cast with 60 or more feet of line out with a 14' rod. I lasted about 6 months with it all.
The only thing I ever caught with it was a Bull Trout that was about 12" long. I was bringing it all in for another cast and I noticed a fish on my line. I didn't even feel it on.
Figuring out the capacity of a fly reel is more fun than doing your tax return, but not necessarily easier. With experience, it gets a little easier. Find the manufacturer's recomendation, though they're not always accurate, or helpful (Pflueger used to give the capacity for their reels with double taper lines - not much help with a 1498). Outside diameter is just a start. Some reels have the same diameter, but with a wide spool model that holds considerably more than the standard model.
Design can make a huge difference, especially medium- and large arbor reels. An extreme "Ferris wheel" large arbor has lots of space within its spindly frame that holds nothing but air.
With that grain range you will want a reel with a decent amount of capacity. If you are looking at a classic spring/pawl reel, a 3 3/4" wide body would be a good place to start. The Abel switch reel would also likely work. For a modern large arbor reel reel, something in the 10-12wt range.
I think that looks good. The key with 2 handers is the balance; you want to be able to hold the rod lightly at the top of the cork and have it sit level with the rod strung up. If it tilts up or down that gets really old when swinging flies, as you're constantly having to tilt the tip up to keep it out of the water, or push the tip down to keep it level.