The 49.2 is listed on the USGS site for E. Fork. It’s listed for each gauge. It represents the watershed upstream of the gauge location. Clearly, many streams don’t have gauges on them though.
As to this particular thread and LVH, yeah, just eyeballing it on a topo map. There’s also some mapping softwares out there that will let you trace the watershed area on a topo map and then give you the surface area. But eventually you get pretty good at eyeballing it. I bet my margin of error is 20% or less most of the time, which for practical purposes is close enough.
There used to be a list published by Lycoming College of each stream in PA’s watershed size, listed alphabetically, but the link I have to it is dead. Haven’t found it if they’ve moved it. Would’ve shared it here otherwise.
I’m a small stream guy, but if it’s less than 5 sq miles or so, I don’t usually bother with a stream, unless it’s really really steep. High gradient streams seem bigger than they actually are. LVH doesn’t appear all that steep on the topo.