Trundle Mattress Size are generally 39 inches wide x 75 inches long. These are standard Trundle size bed measurements. Trundle Size Mattress depths can vary from just a few inches to up to 8 inches thick. Keep in mind that with bedding, the trundle mattress must be thin enough to roll under the bed. With a trundle mattress, it is always a good idea to use the same thickness mattress on the top bed as well as the trundle so when pulled out, the trundle mattress will be the same height in case you wish to push the two mattresses together to make a short king. Many times, actual trundle mattress dimensions will vary by one or two inches, usually smaller.