how to get the size of a structure by looking at address

If you have the source it is trivial. Use sizeof on the dereferenced structure address. But I suppose you are asking when you don't have the code.

The only way to get the size of the structure is to analyze the memory content. Even there you must have a fair bit of idea to separate the "junk" from the data. In debug mode, certain memory addresses seem fixed and things get loaded there quite frequently. If you can, first get to know the address after a few dummy debugs. Then load a dummy array of a suitably large size in that area, delete it and then load this structure in that area. The extra CD (or whatever) bytes must be able to help separate "junk" from the structure's data. This isn't fool proof since the data of the structure might coincide with the bytes.

Yet, in spite of these efforts, all you will get to know is the size of the entire structure. I mean you won't be able to make out if the structure has an array of eight characters or four shorts or two integers or just a double. It isn't impossible but you have to do a lot of dirty work in decoding the raw bytes.