Since sizeof(char) is 1 click on the 'byte' option and thats how many bits are allocated in memory.
sizeof(char*) is 4 so click on DWord and thats the bit allocation.
R0mai says you're probably running on a 32bit machine because pointers(char*) are locations in memory. So if the size of that pointer is 4 then it can only address up to 32 bits. If you were running a 64bit machine/executable then it would be 8 so it could address all 64 bits.