I usually put the part in PCB and zoom right in until it’s huge, and move the cursor from one side to another and read and subtract the x.y cord in the bottom right - click on it to change units -
If you want more accurate you right-click on the part and select Edit, goto PCB, then File/Show in Folder and save that svg. Then open it in Inkscape and use the ruler.
You can also print the part and see if the part fits on it, but it’s hard to see small legs.