There is no guarantee that when you build a library on a 32-bit platform, an int will have the same size_t as an int on a 64-bit platform. They usually will, but there is no guarantee. A simple example is this:

sizeof(char) == 1 
sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long) 

Therefore, a short can be the same size as a long! More usually, though, int will be the platform word size (32 bits on a 32-bit processor, 64 bits on a 64-bit processor).

Values for floats are stricter and comply with the IEEE 754 standards.

There is not normally an issue if the Rust application is built on a 64-bit platform and the library is 32 bits. If it is the other way around, however,there is a chance that an overflow may occur. It is unlikely, but worth keeping in mind.