Replacing Electric Built In Oven
Typically in houses, ovens are wired using #8 or #6 copper. Newer ovens usually have #10, sometimes #12. The reason is because appliance wire is rated for higher temperature, so it can be smaller.
If you use the big blue wirenuts, they will accept a #6 and a #10. In fact, they'll accept 2 #6's. The big grays will take a #8 and a #10. When making the splice, make the smaller wire stick out past the end of the bigger one by about 1/8". Twist it pretty hard. When you're done, hold the wirenut with one hand, and try to pull each wire out. Pull hard. If it won't come out, your splice is good.
I know this sounds easy here, but when working in cramper quarters (like an oven cabinet) it can be a bit of a challenge. With a little patience though, you'll get it.