Depending on the NAS, you may not be able to increase your existing volume above a threshold, but it is usually possible to create a new volume. Specifically for your case, I found the following:
https://forum.synology.com/enu...
so: if you are running a DS1010+ with DSM 3.0, you have to evacuate all data and create a new volume to have a larger volume than 16 TB in your NAS. but i would advise you to upgrade to DSM 5.2 first anyway (runs much more stable in my past experience). I can't reproduce it 1:1 at the moment, because I'm on DSM 6.x with my NAS.