This really depends on what type or types of files you are storing. If you are storing text files, spreadsheets, emails and financials the chances are you are going to need less than 2 GB of storage.
Consider these approximate figures below. Note: This will vary based on quality and length and should only be used as a rough guide.
Average mp3 File size; 3MB
Average movie File size; 700MB
Average HD movie File size; 5000MB
Average Photo File size; 5MB
Average text File size; 0.2MB
Average spreadsheet File size; 0.5MB
So you can calculate roughly how much storage space you require. If you have large photo albums but no music or movies then maybe you will need a 100GB Plan, if you have lots of movies we highly recommend using an Unlimited provider.
You should always consider the future when you are signing up for an online backup provider. If a provider only has plans upto 100GB and you currently use 80GB you should look for a provider who has plans which can accommodate your needs in the future. This is because moving providers is a long process. You have to download and re-upload all your data again, which could take days/weeks.