I have a project that stores all the class declaretions on one header i know that might increase the size of the exe and is not recommend if you are going to let other people use the code, but i am willing to live with that my question is are there any major issues that i should know other than the two i mentioned. and would inserting #ifndef and #define between class help.

it won''t increase the size of the exe

I''ve similarly wondered about things like this.

Does including extra headers or extra code that you don''t use affect your compiled program at all?

For instance, if I don''t define WIN32_LEAN_AND_MEAN, does that really change anything besides the time it takes to compile?

Compiler only includes code that is used in the final executable.