how do i know what size lathe to get??

In my experience, start with a mini and work up, or save yourself a lot of money over the years and get the biggest lathe that you can fit in the house without displacing important family members..

I'd recommend something at LEAST the size of my lathe, (jet 1442), if not bigger... The "42" sort of means what length of stock you can turn, as in 42" long... Good reference, until you TRY something close to the max. length and realize that the measurement is for the whole bed length and you need to subtract a bit off that for the head stock, tail stock, etc....

I just went out and did a rough measurement and I'd be pushing it to turn over about 32"... I guess that unless you buy a really huge lathe, you're looking at 1 or more bed extensions...