Those are meant to be cut to size.
 
As to the thickness I'm not really sure. It's be hard to have tapes that are too thick as they can be smushed down with heatsink pressure, but too thin for poor contact is a real possibility.

you can check with a caliper   and use on the uncompressed /undamaged part of the old pads/ tape .. then ''your''  more sure your right 
 
https://www.amazon.com/Best-Sellers-Industrial-Scientific-Digital-Calipers/zgbs/industrial/2476630011
 
https://www.amazon.com/Pittsburgh-00895-Digital-Micrometer/dp/B007L1TUZC
 
maybe a local shop or auto parts will have the tool to rent or just take the pads to them and they will help you check the thickness ?
 
or a feller gauge to check the gap between the cooler and the parts of the card the pads go 
 
 
I guess it would be nice if evga sold a thermal pad kit for common lines of cards   like the offerings for the 10 series deal kinda thing ?

no contact  for it to cool properly   the heat got to transfer up and out   a gap there breaks that   . with older card the pcb can warp a bit  and even the correct pads maybe off a bit as well  to where maybe a extra .05 in thickness may be better ?
you can compress a tad too thick but you cant make up for too thin   there is a limit to what ends up too thick 
 
you want the best fit for the most efficient cooling