Across the political spectrum, considerable anger, frustration and range is directed either at big business or at big government. Clearly, the thing that unites these viewpoints is the presence of inhuman and impersonal scale, together with the inequitable concentration of power that this scale implies. For comedy effect, I shall conflate Mammoth and Mammon and term this fear of large organisations "Mammonthophobia". To this fear of the very big, I would add a second phobia, that of long-lived organisations; (Geriatrophobia?) for corruption comes to us all, and the longer lived an institution, the more deeply embedded the rot. The Peter principle captures the gist, but fails to convey the seriousness of the message, nor the sinister nature of the disease.
Other commentators have pointed out that size is difficult to measure. This is true. To these cautions I would add the ever-pertinent one that legislation breeds corruption. Too many people already make a living by exploiting tax loopholes & legislative weaknesses (Private Equity, I am looking at you).
Placing these practical concerns to one side, I think I would be rather more radical than you in my goals. Team sports give us a good idea of what a human-scale team looks like - between 5 and 20 individuals. I am less sure what the longevity of a business should be: between 2 and 8 years seems like as good a guess as any. The ability to invest in the long term and to take on big projects is less inhibited by these restrictions than you might think: even the largest of today's businesses still need to raise outside finance and work together with other businesses to solve large problems; placing severe restrictions on the size of businesses will only help to increase the level of automation and systematic discipline when it comes to organising the crowd of small business-teams to take on large projects. In any case, we could hardly do any worse than our current economy on long-term planning and investment.
I am not sure how a government could encourage the formation of an economy formed largely of small, young businesses, I am only sure that it should.