Depends on how much you lower it. I've got lowering springs, with 1 more coil chopped off on each corner (so it's been lowered by 90mm), and I run 15 x 8 deep dish alloys on 195/45/R15 tyres, with a bit of tyre stretch and no camber. I couldn't go for tyres with a bigger profile, but I can get wider tyres without it scrubbing too much. And if the alloys were any bigger, I'd need flared and rolled arches.
The smallest size alloys you can go for are 13" (otherwise they won't fit over the front calipers, and rears if you have them), but they can be any width as long as you don't mind getting some bolt on arches to stop them sticking out. I think most people on here tend to go for 13s, 14s, or 15s. Any bigger, and your speedo will start to be out.
With regards to tyres, you can go for pretty much any size, depending on how low you want to go, and what size your alloys are going to be. ANd the 4-spoke SR alloys are nice, they tend to come with 175/50/R15 tyres I think, I had a set on mine before I got the Calibres. That's the main reason I stuck with 15s.
Front splitters are also very easy to fit, especially the Polo ones. The bolt holes on them line up perfectly with the ones on the Micra bumpers, so pretty much no drilling is required.
It's also a case of looking at other peoples Micras, and seeing what you like and what you don't like
