One area is the native size since it really is an 8 bit processor, but because C limits an int to be no smaller than 16 bits, an int is 16 bits on the AVR. The AVR has many issues when it comes to C. I say normally because this doesn't really hold for the AVR. When size doesn't matter, and the value is known to fit in 16 bits then int is normally used. These days, if size matters, then you should not use native data types and use types from stdint.

Prior to that you had to play games and define your own types to get known sizes for your data types. The stdint types (which is what we are talking about) were formalized in the C standard about 20 years ago. (I saw one for the 6502 back in the very early 80's but those days are long past.)

So there is no C implementation where an int is 8 bits. The C standard has required the minimum size of an int be 16 bits for many decades. This makes it 8 bits for 8 bit architectures, 16 for 16, etc. Just using int causes a huge number of issues when you move from one architecture to another, as the size of int is usually defined as the 'natural' size for the architecture.