If you are using Veeam ONE with MS SQL Express you probably either have a small environment or you just didn’t hit the 10 GB database limitation yet 
But, how much space will the database then consume?
The good news is Veeam offers a Veeam ONE database calculator tool right here: https://www.veeam.com/kb2246
It is actually a Excel spreadsheet, but is very easy to use:
But wait! Don’t forget to select the correct data collection mode you plan to use at the bottom:
In case you are wondering what this is, check out the user guide: https://helpcenter.veeam.com/docs/one/deployment/typical_choose_collection_mode.html?ver=100 and the table from https://helpcenter.veeam.com/docs/one/deployment/data_collection_modes.html?ver=100 (partial screenshot):
