Hi,
I've just purchased a 1998 740il from a private seller. He installed 18" wheels. I need new front tires and have discovered the back tires are larger than the front. Is this normal? Should I have all of the same sizes?
The current front tires are: 235/50ZR1897Y
The current back tires are: 255/45Zr1899Y
All Yokohoma brand.
What brand is recommended or best?
PJR

It is normal, don't worry about it.
The downside about this kind of setup is you can't rotate the tires around. (wider tires in rear gives better traction for the driver wheel)
You might want to go to tirerack.com and see the matching tires and read some owner-reviews.