Convert your crib to a full size bed by ordering a full bed conversion kit.  The full bed conversion comes with extra solid wood slats that will be spaced out 3" apart so no boxspring will be needed.  Once you convert the crib to a full size bed then you will need a full size mattress (54"x75"). 

Construction:
Made from local sustainably-grown trees.
100% solid wood construction.
Available In solid brown maple, red oak, hard maple, cherry and walnut woods.
100% solid wood slat mattress support made from unfinished brown maple wood. (No Metal Springs).
No plywoods, no MDF, no veneers, no engineered wood, no composite wood, no particle board anywhere.

Meets and exceeds all applicable federal standards and regulations
Consumer product safety commission (CPSC)
American society for testing and materials (ASTM)
California Prop. 65: Tested for lead, cadmium, and phthalates.