Somebody else will have to chime in with which tub sizes interchangeable as far as being the same height/how many fit side by side etc... because I'm the same way, I have to see the dimensions in front of me to know how big things are. My boyfriend laughs at me because I measure with parts of my body when we are at the hardware store. I can accurately tell which screws or parts I need by using the thickness of my fingers, length between joints on my fingers, or the distance from my fingertips to my wrist or elbow