One of the size 50 wide spool lever drag type big game fishing reels will hold around 850 yards of 50lb mono, while a narrow spool version will hold around 550 yards of 50lb mono.
Conventional big game fishing reels are sized according to the original Penn system, from 6/0 through 9/0 and 12/0 to a mighty 14/0. Typically, a 9/0 will hold around 675 yards of 50lb mono and a 12/0 around 800 yards of 80lb mono line.
Here in the UK it's probably only sharks that will test big game fishing reels to anything like their full capacity, but elswhere in warmer oceans the super-fast and powerful gamefish - large billfish particularly - will test them to the limit.
Any fishing reel, but especially a big game fishing reel, will be the most complex mechanical item - and likely the most expensive one - in your tackle box. As such, it will pay in the long run to choose with care and buy the best you can afford.