I've got those same 7-hole wheels... the stock tyre size for these wheels is 175/65/15.
An alternative equivalent tyre size to the aforementioned 205/55/15 could be 195/60/15.
15's are a bit of a pain due to the limited availability/choice of performance tyres.

Insurance companies (at least MINI Cover, i.e. UK Insurance Ltd) don't seem to understand tyre equivalence and won't let you run on anything except the stock standard BMW-approved 175/65/15 with a choice of either: Dunlop Sport 01 (terrible in cold and wet, I hear... this worries me.) or Pirelli P3000E (I have these, not satisfied with performance or wear).
Would really like to use tyre equivalence to get a set of Goodyear Eagle F1 DS-G3 tyres, or Michelin Pilot Exalto PE2 tyres - but insurance is a killjoy.
