Which is better, a bar or a ring?
A bar to begin with it allows the fistula to heal straight, you can then switch to a ring if you choose to. Getting a piercing and allowing it to heal with a ring in can then become an issue should you wish to switch to a bar as the hole can heal wonky. This is very common with navel piercings, that are typically pierced with a ring, upon switching jewellery the piercee will find the jewellery (bar) won't sit 'straight' and will instead hang to one side, itms.
Cleaning shouldn't be a problem with a bar provided the piercer uses the correct size jewellery to begin with - it should be a few millimetres longer than the fistula (ie, there'll be a little gap that'll 'stick out'), allowing you to slide the bar from side to side to aid cleaning (and don't use commercial alcohol-based sprays, warm salty water soaks will do just fine) and also allowing for swelling in the first couple of days.
A too-short bar will not only make cleaning difficult, but can lead to some pretty major complications if it's short enough to pinch and interfere with healing. Just a guide, if your nipple is (eg) 10mm across, you'll probably want a 12-14mm bar placed in initially, and once it's healed you can then switch down to an 10mm to fit.
Good luck, HTH!