The definitive way to measure molecular size is X-Ray crystallography. This gives you the structure of the crystal including the positions of all the atoms, so you automatically get the molecule size. This method works for any material that you can crystallise even including huge molecules like DNA and proteins. in the gas phase the size of molecules can be easily measured for simple molecules using rotational spectroscopy. This gives you the moments of inertia about various axes and from these you can calculate the bond lengths. The method doesn't work for molecules that are too big and complex or (like succinic acid) that aren't easily vaporised, however the bond lengths are pretty constant across most molecules. So once you have bond lengths for simple molecules you can put these into molecules like succinic acid and calculate the size of the molecule that way.