How do I determine bottom bracket size I got a cheap old bike I fixed up and I want to install a mid drive motor but the ones I found require a specific bottom bracket size and I don't know what size bottom bracket I have.

The wheels are 700 x 23
I found this diagram online which I hope will allow me to determine the size of the bottom bracket by measuring the width of the shell.





I found out that the bike is an old Centurion.  I was not able to find any series number engraved on it anywhere so I haven't been able to determine the exact model or how old it is but just in case anyone wants to know; I concluded that the bottom bracket size is 125x 68 but that a 127 size also works.  Thanks to the mechanics at IMartin in Los Angeles who took the whole crank assembly apart to measure the bracket when I asked them if they knew what size it was.