Well, you're definitely not going to be able to use anything plastic-y for the heat, so you might be stuck finding a huge pot
But, they really don't need to be too big - a stew pot should work if you do each pair of pants separately. If you go to a discount store or a thrift store (or somewhere like Goodwill/Salvation Army) you should be able to find a pot to use really cheap - just be sure that you don't use the pot for cooking afterwards, it's not a good habit to get into, because you're using chemicals to dye, and you don't want to be eating that stuff
And, you'll need to do each separately - there is a chemical reaction that takes place to set the dye in the fabric, and once it's done, it's done - you can't dunk another pair into it, it just won't stick. So, you'd have to empty it out and start again
Oh! Don't wring fabric - it's best if you take an old, icky towel and roll it up like a swiss roll to get excess moisture out, but that's more of a case of after it's somewhat dry...