The stands commonly used are the old square bases (25 x 25mm), the newer strip-style bases (10 x 40mm) and round bases – commonly a 2p piece (which is ~22mm in diameter).
Unlike Epic: 40k
, where there were shenanigans possible with basing, there is no advantage or disadvantage to using any particular type of base, so go with what you think looks best.
Sand and flock commonly used in 28mm-scale basing tends to look a bit coarse at this scale, so I recommend fine flock or fine/extra-fine grade ballast, available from Hobbycraft and other local model shops.
Remember – as long as the stands follow the rules in 1.1.2, you can have them whatever shape you like – even Legion symbols or something equally esoteric!
Note also that you can have between three and seven infantry models on each base. This makes cool little dioramas possible (and awesome)
* Available as a free download from Games Workshop. CLICKY LINK