[Anthony]

Actual mass flow rate is very unlikely to be an issue for surface area, though it can cause some design issues for pumps. The problem is that you need enough area for your exhaust gases to expand properly. Thus, the 12' wide end of the nozzle on the Saturn-V mentioned below really did matter, and even a fairly modest increase in size would run a risk of running out of space on the bottom of the rocket (unless you came up with some other form of technological advance).

Non-chemical rockets will generally have problems with the surface area requirements for heat management and power generation before the actual rocket size becomes a problem. Solar and magnetic sails (and solar cells) don't scale up well at all.