For a 1A Toolmaster mill, 1hp. I actually have a 3/4" collet but it's galled up on the inside and I don't think I trust it. I assume a Toolmaster can run a 3/4" endmill since I've often heard that's the upper limit of a standard BP (and these are supposed to be a bit stouter), and the 1" is more for large facing tools since it's such a round/common shank size. The mill came with a 1-1/8" collet and I actually had a cutter with shank that size magically, but I think the collet has been tightened hard without tooling in it because it won't get on that 1-1/8" shank for anything.
I also wonder why they made them with such wide spindles and collets if the machines won't run those diameters, but maybe that's just ignorance.
What do you find your limits on a 1D to be, Daryl? Thanks for the response.