15" x 5.5 is about as big as you can go without rolling the guards, the only standard wheel I know of in this configuration is the old bluey TRX rim.
For ages I tried to get my local wheel place to find some aftermarket rims in the same configuration, but they didn't seem to be able to (unless I was spending over 2K). In the end I gave up & went for 14" Skyline steel wheels & painted them...... Less unsprung weight too!