15" wheels were standard on the M3 and the iX.  14" on most other RWD E30's.

14" stock size is 195/65/14
15" stock size is 205/55/15.  Let's use this as our baseline

On my iX I did a +0, to
225/50/15.    -0.1% circumference.

For a 16" wheel you could do:
205/50/16 = +0.8% circumference
215/45/16 = +1.1% circumference
225/45/16 = +0.4% circumference

in a 17" you could do
205/45/17 = +1.6% circumference
215/40/17 = +0.4% circumference
225/40/17 = +0.9% circumference

My "+0" of 225/50/15 gives improved grip and cornering, increased
tramlining, stock ride, essentially no speedo error.  This would be quite a
change already from stock 195/65/14 size tires.  I don't give up anything
over stock except some tramlining, but I gained handling.

Every time you go up in wheel size, you go down in profile.  So, you have a
tire with shorter, stiffer sidewalls.  This gives improved cornering due to
sidewall flex control, and reduced ride quality because vibrations are
transmitted with less dampening.  As you head down into 45, 40 even 35
aspect ratio, your short sidewalls begin to expose your rims to damage from
potholes, and your ride harshness increases.

When you go wider, you improve cornering and braking by changing the shape
of the contact patch.  Firmly inflated tires will still maintain a good
sized contact patch and deform less than tall tires will, so more rubber
stays in contact with the road.  But the wider they get beyond stock, the
more they want to follow the grooves in the road.  As you move up in width
eventually they will begin to rub the body or suspension, but I have fit
225's in my car with no rubs.  You may need to roll your front fenders
slightly.

Lastly, as you go down my chart above, you'll find that they line up almost
perfectly in this:  Farther down costs more.  Both for wheels and for tires.
Lower profile costs more within the same size also.  Hop on tirerack.com,
pick a tire type, and look at the prices across the size ranges.  Pick out
the sizes in the chart above, and compare prices. Do the same for wheels.

Many of the guys in the lists that wear 17" rubber aren't that pleased with
it -- these are bimmer guys not ricers, and they don't like bending rims and
rattling their kidneys.  Also, larger rims weigh more, and unsprung weight
is bad for autocrossing, so many of the guys autocross on 15" wheels.  The
16" guys are mostly happy with a few grumbles here and there, and the 15"
guys all seem happy.  The 14" guys are envious of everyone.  Except we
pretty much all wear 14" tall skinny tires for winter if we have heavy snow
to deal with.  :-)

So, decide what your priorities are, and what your budget is, and have at
it.  All the sizes I listed above are truly minor variations on your
speedo -- I wore the 205/60/15 for a while on my car that it came with,
which are 3.4% too big in circumference, and it was no big deal for speedo
error.  Only under hard cornering (ie, 2 track days), they would deform
enough to rub the shock towers, and they would also rub if I hit a bump at
full lock in a parking lot.  And they handled like crap, they were
all-season.  Yuck.

-Russ.

I run 205/55/15s and am very happy with them. 17s will ruin the ride on
an E30 and it'll tramline all over the palce - avoid.