Good quality bed for an affordable price.  Just what I needed for my son’s “big boy” bed.  
I waited to buy the mattresses, until we got the bed assembled, due to some previous reviews and the unclear product notes.  The dimensions of the trundle were unclear, so I didn’t want to make any assumptions that the top of the bed would accommodate a regular twin mattress.
For those in a similar situation  
The length of the trundle bed is 72 34 inches (to have some room to spare) and you definitely do not want to get a mattress that’s thicker than 6”.  
As for the top bed (twin), a regular twin mattress will fit (76” x 39 14”).