The inside dimensions are 8.5"wide x 6"deep x 6"high. The standard sign holder is 5.5"high above the box so an 8.5"x 11"sign will fit in the sign holder but only ½ of the sign will be behind the body of the box. The slot size is 4" x 7/16". It is large enough that this can be used for drawings and large enough to add a brochure pocket. Brochure pocket 4"x4"x 1" is an additional charge of $4. An optional sign holder that is 8.5" x 11" above the box an additional $3 is added to the prices listed.