I've not done multi-machine rendering for a while, but the last time I did I was using a 2.4GHz Quad, a 3.4GHz P4 and a 2.4GHz P4.  I found that with a larger tile size the faster two computers were generally finished and waiting around doing nothing whilst the slowest was still rendering its last tile.

So I'd have to say that the ideal tile size is dependent on the comparative speeds of your computers.

I do remember being a bit annoyed at myself on one render I did between just the fastest two machines - it was actually a few minutes slower on the network render than it was just on the quad alone - when I messed about with the tile size it got better, but for me it was a case of trial and error.

i7 5960X, 32GB RAM, GTX 1080Ti, GTX 980 Ti, Windows 10 Professional.  Running Daz Studio 4.11, Poser 11, Vue Inf 7, Photoshop CS4