Is the problem wrinkling or curling? All papers curl with albumen...the lighter the worse.

If you want to go really big, you really should try a heavier paper. Lenox 100 would be suitable if you don't mind the texture. If you want something a little smoother, you can use just about any of the heavier papers by Arches.

Something else to consider with large albumen prints is the volume of albumen and silver nitrate required. You'll need about a gallon of albumen to sufficiently fill a tray required to float an 11x14 sheet.

Yet something else to consider is that with albumen I've always found the biggest obstacle to be air bubbles. The larger the surface the greater the potential for small bubbles to ruin your finish.