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To be clear, I know nothing about fishing.
On another project, someone told me he used 50lb fishing line, and when I looked it up to buy some, I found it said this: "Premium Mono Line 1/4Lb Spool, Clear, 50Lb, 250Yds" (https://www.westmarine.com/buy/ande--premium-mono-line-clear--P0368045?recordNum=1)
My question is: what is that "1/4Lb" mean? And how do I find the diameter?
One quarter pound is the weight of the spool. Using the option to change products on the website you'll see that the line comes in a 30# weight (400 yards per spool) and a 50# weight (250 yards per spool). Notice how both spools are the same size and weight, with different lengths of filament.
Things probably seem less odd when you consider that a fishing reel is constrained to a certain volume of line; the heavier the line the less you can wind on the spool. Given that it's unreasonable to assume anyone will know how much line of any given diameter will fit on their reel it's simpler to sell bulk line by weight. There's probably some attendant cost savings with having all of your bulk rolls be the same size too.
Fishing line is marketed by breaking strength, not diameter. 50# line made from different materials will have different diameters. You should, however, be safe substituting one 50# monofilament for another. Any size differences are going to be very minor. Think aluminum foil, which may vary in thickness by brand without affecting its use in craft projects.
I did a quick google search and listings for the same brand and test-weight on Amazon are listed with a title saying something along the lines of
ANDE Premium Monofilament - 1 lb. Spool - 50lb. Test - Clear
or
ANDE Premium Monofilament - 2 lb. Spool - 50lb. Test - Clear
Which shows that the 1/4 lb is a quarter-pound (weight/mass) spool for bulk fishing lines (note the subcategory for the listing in the OP's link is: Fishing /Fishing Line & Leaders /Fishing Line / Bulk Fishing Line), which presumably translates to a certain distance for that particular diameter line. Presumably a lighter test-weight line with the same weight spool would have a longer amount of line on it due to diameter being smaller.
1/4 #not1/4 lb. I presume the spools are made in standard sizes to fit different dispensers. So the spec is spool size, line size, and line length.