Perhaps it would be helpful to know which TW game you´re talking about, Breeze.

In MTW there´s a limit of 1260 men per army, in Rome the absolute maximum is 4800, as far as I know there is no limit to the number of men per army.
The unit size can be changed only at the beginning of the campaign, in the graphics options. Once the campaign is started this setting will be valid for the whole campaign, even if you change the settings.