All interior crib dimensions and mattresses sizes are regulated by the CPSC, we are required by law to adhere to these standards.
Due to the varying density between mattresses, some will fit more firm that others. By law, a crib mattress is required to be no smaller than 27-1/4” x 51-5/8” when measured from binding edge to binding edge. Depending on the manufacturer and style of the crib, the interior of the crib can be between 27-3/8” and 28-5/8”wide, and 51-3/4” and 53” long, so there can be some space between the mattress and the inside of the crib.
For the snuggest fit, you will want a firm mattress. Firm mattresses are not able to compress as easily and would serve to fill the space between the cribs. Coil mattresses are generally more firm than foam mattresses. Although another mattress may be 27.5", if it is denser, it will fill the space better.
Parenting websites and blogs recommend the two finger test. The space between the mattress and side panel should not exceed two finger widths.
For the best fit in an MDB Full-Size Crib, we recommend using the Emily II, 2 sided 260 Coil Mattress.