A 14 is very likely to work just fine or you can put a decorative board behind the tank.

Since the distance right now is 13.5 inches from the center of the drain to the wall, do you think for the 14 inch rough in toilet that the tank would end up too close or resting against the wall? Thanks.

Some toilets are a little closer roughed than others... but a 14" rough will most likely work here (as Doug238 suggested)... it will be tight.. could even hit the wall, but will depend on particular toilet purchased.

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