As every other person mentioned, you can use compiler options to reduce your size. At first, try to tweak these options for better result. These options normally affect size of your code.
But if you have a lot of resources in your EXE/DLL, you will not see much difference. If you really need a small size in this case, I suggest you to use a PE-packer. A very good free PE-packer is UPX.
UPX is an advanced executable file compressor. UPX will typically
reduce the file size of programs and DLLs by around 50%-70%, thus
reducing disk space, network load times, download times and
other distribution and storage costs.
You need to run upx as a post build process to pack your EXE/DLL file with a command like this:
upx --best mydll.dll
PE-packers compress your code and resources and encapsulate them in another EXE/DLL file. Then these files will be unpacked at runtime automatically, so you can use them like a normal EXE/DLL file. Even though PE-packers compress codes too, they are super effective when you have a lot of resources in your EXE/DLL.
solved What’s the best way to make DLL size as small as possible?