Hi, BeachBum, and welcome to the forum!
I use the glass measuring cups to melt down my base and I like them just fine. I have a 1-cup and two 4-cup glasses and those suit me for now. I use the 4-cups the most, and the 1-cup if I'm melting only a couple of ounces. I can see where an 8-cup glass would be useful if you were making a bunch of bars at once or a loaf that doesn't have many layers. I'll probably end up with some 8-cuppers someday.
Oh, and I put plastic wrap over the tops of the glasses when melting the base in the microwave to help keep the moisture from evaporating.
As far as how large a typical batch is, I guess it really depends on what you're doing. A pound of MP base will get you approximately four bars, assuming each bar weighs in around four ounces. Most mold suppliers will tell you how much each cavity holds, or you can measure it by pouring water into the cavity and then weighing or measuring the water. (A scale is way helpful -- a cheap food scale is fine as long as it is fairly accurate.) A loaf mold like the one Bramble Berry sells holds about 2-2.5 pounds. So, you just multiply however many bars or cavities by however many ounces each holds, and come up with a number. If you're layering or something like that, there's a little guesswork concerning how much of each color or whatever you'll need for each layer, but if you know what kind of layering you want to do and you know how many total ounces you need, you can divide it up approximately in your head and be okay.
Hope that helps. Have fun soaping!