Hmm.. are you driving a 2000? They put some little wheels on those bad boys.
As far as I know though, the wheel hub is the same on all echos, which means that it should accomodate a 15" wheel without difficulty.
Some tips though.. the stock 15" steel wheels are 15x5 45 offset, the 14" alloys are 14x5.5 45 offset. So even with Toyota OEM wheels, there's some variation in wheel width and fitment. I think you'd be safe in looking at comparable sizes. Here's a wheel offset calculator..
http://toy4two.home.mindspring.com/offset.html
The problem you can run in to with improperly sized wheels would be strut or splash guard rubbing. If you run too big a tire, it can rub on the arch as well and accentuate any strut clearance issues.
Hope this helps!
