I lowered my truck 3' with djm dream beams, i have 275/60/15, i was wondering what a good size not to rub would be

 

Each truck is different. especially when you customize it. You will have to do some measuring, and then hunt around on the web and you should be able to find a site that translates the tire numbers to the actual tire size.

 

Don't trust a calculator or a website!! Actually go to the tire store and measure the
tire and hopefully you can measure it while it is on a rim of the same width you will
be using since from manufacturer to manufacturer there are variances in tire size and
rim width plays a big part in how the tire will measure out! bring your tape measure
and a piece of chalk, Mark the tire and the ground roll forward until the tire mark
meets the ground, remark, measure from pavement mark to pavement mark that is
the measurement you want to compare your tire selections to.
https://www.ford-trucks.com/calculators/index.php here is some calcs to use for your
selections.