i just got an 80 schwinn super le tour i'm building up as a project. and i'm realizing i have no idea how to figure out what size BB i need. is there a rule of thumb or some way of going about doing this? TIA.

What cranks are you planning on using? Someone might know off the top of their dome what bb to use.

The simplest way is to throw in a bb you currently have (assuming the taper and shell size are correct), install the cranks cog and chain, figure out how far it's out and go from there. You just have to keep in mind that you need to multiply how far it's out by 2 to get the correct difference in spindle lengths (since bb spindles have two sides)... BUT you also have to take into account that sometimes a bb spindle one size up from another is only longer on the drive side.

I think Sheldon has a bb size database that may be of use to you.