Hi nickofen !
*Everything* depends on your goals ..
and capacity and productivity depend on rigidity above all else.

Ex.
I could cut about 0.1-0.2 mm deep with a typical setup, using a 50 mm wide 4 insert apkt facemill.
(12" Lathe, upgraded ac servo spindle 2.5 kW continuous.)
I built a modular vice using 4 pieces of 50x100-300 mm solid tool steel billets, cross-drilled for 16 mm hardened industrial grade 12.8 bolts.
Vice took 3-4 days to do, and the 4 parts mass == 50 kg.

I could then cut the same piece, about 200x200x80 mm, with 3.4 mm deep cuts at double the rpm.
The difference in productivity is about 10:1 - 20:1.

I don´t think there is a "right" answer, but vastly bigger and heavier and thicker is usually the right choice for *everything* in lathes and mills and routers.

They deliver about 20x the performance, typically for 100-150€ more in steel costs and 2 days extra work in assembly work and fitup, per assy.