So I am planning to make my first DIY diorama and need to be working off of pictures. This rodeo has probably been done many times already so my question for those of you with experience is does anyone know of a reliable method to determine distances and sizes working with pictures?

Hello Willem,
Doors seem to be a standard height or armor, tanks of all sorts have standard dimensions find the height dimension and take your measurements then do the tricky mathematics.
HTH !
Tom