Unit Size & Ancillary System

[Raygereio]

1) I play on large and both my computer and I are happy about that.
Actually I found cavalry to be more devastating on smaller settings, on especialy huge the infantrygroups have more men to 'absorb' (read: get run over by) the charge and don't rout as quickly.
Personally if your computer can handle it; try huge, if only for the epic-big-army feel.

2) The imposter-bug; I assume you're referring to the whole "All my FM's get the allied general-trait!" thing? That's caused by the fact that EB1.2 is not save game compatible with previous versions.
Well, that and people not reading properly as this information is given right here.