As already pointed out , your trying to copy a "full frame" slide on a "half frame" camera .
Or thereabouts .
As your slide copier was designed for one film camera to copy another piece of film it was never designed to project the full frame into a small area of the film used to copy it as it would leave a lot of blank film around it .
If you want to use this device to copy film at 1:1 then you need a full frame camera .
Depending on budget , if you want to stick with Canon , you might want to look at an original Canon EOS 5d .
typically these sell for around £200 .
Also , as this is an analogue photography forum only , you might want to ask more about digitalising film in the Hybrid photography forums , in particular the scanning and scanners sub-forum , where there is already a lot of information on using DSLRs to digitalise film .
https://www.photrio.com/forum/forums/scanning-and-scanners.361/