20 by 20 in 6 mm will be a pain.  Lots of little bases to move around but without the handle of a 28 mm figure.  You will need to use a unit base which then defeats the object of the small base in the first place.  Hail Caeser doesn't worry so much about individual models so 60 by 30 can represent the limited formations you may have.  You can always go bigger by having a standard unit 2 bases, then 3 then 4 for a frontage of 240 mm.  Four bases with 24 figures on is pretty much a maniple. 

Thank you, I will consider that.
And what bases for cavalry and litgh then? :/ 50x25?

 The idea of using very small bases for 'flexibility' sounds great in the abstract.   The problem is that in any game, the more individual bases that you have to move, the more time is taken up.  In really big games this can be a massive problem.  Also, as has been said, 20x20 bases can be a tad tricky to handle.

Hail Caesar and its stable mate Black Powder are very popular with 6mm gamers.   There are two schools of thought with regards to basing.  Both of these accommodate the standard strength unit included in the Baccus army packs.

• A base size of 40x20.  This is a hangover from the older, but related Warmaster system by the same author.  Three of these bases constitute a 'unit'.   Each holds 4 strips of close order infantry (16 figures) or two strip of close order cavalry (6 figures).    These allow for a small, medium or large unit.
• A base size of 60x30.   Two of these bases are a standard 'unit'.    Each holds 6 strips of close order infantry (24 figures) or three strips of close order cavalry (9 figures).    These allow for a small or medium/standard unit.   To be honest, these two sixes will work for all the armies that we supply.

Naturally, I prefer the bigger bases.  Far less fiddly, and the extra real estate allows you to really present the finished units nicely.

Does that help at all?

Dear Peter... it helps as usual... but not entirely... basing is a tricky thing... I have no complains with using 60x30 on my ACW Armies BUT the Cavalry!... heavily wooded terrain... straight BRIGADE line?... OMG!... one of this days I will start a thread explaining why ACW Cavary does not suits 60x30 basing as a rule (POLEMOS ALTAR OF FREEDOM or whatever...). The only valid rule is to each his/her own and ask for them at PENDRAKEN (Leo is helpful)... I can understand that Peter can not stock all the variables in dimensions and sticks to the well proven 60x30 or 30x30.  Lets enjoy ourselves without dogma!