Depends what order you want to route; if you want to route the mics into your PSR, then I'd get an MG06x (no x means no reverb, which is kinda necessary for singing but not for talking)l or a Mackie Profx8; any small mixer with 2 xlr inputs and fx will work. But as Paul suggested, the AG06 (now mk2) is a great choice because it's got 2 mic inputs *and* fx *and* a built in 2 channel interface, as does the Yamaha MG10xu, but that has 4 mic inputs.
However, if you want to route *into* the mixer, and add the mics in parallel to your keyboard (as opposed to into your keyboard) then skies the limit, although you'll then need to invest in live pa speakers as well. But in that case, you'll want more channels rather than fewer. Again if the mics are used for singing, you'll want the version (whatever mixer you get) with an effects processor. The AG06 mk2 would be especially useful here as you could record everything into your pc via USB. An MG10xu would also work, but might be larger than you want.
Mark
