How to determine how much storage to store backups?
I believe in order to get the calculations correct to somewhat close to an appropriate figure would be collecting a data or a trend analysis as mentioned by Kin. There are various methods available to gather such data:


One approach would be analyzing a trend for the size of backup that has happened with its history being retained in msdb backup set from below query:

SELECT
[database_name] AS "Database",
DATEPART(day,[backup_start_date]) AS "Day",
AVG([backup_size]/1024/1024) AS "Backup Size MB",
AVG([compressed_backup_size]/1024/1024) AS "Compressed Backup Size MB",
AVG([backup_size]/[compressed_backup_size]) AS "Compression Ratio"
FROM msdb.dbo.backupset
WHERE [database_name] = N'Databasename'
AND [type] = 'D'
GROUP BY [database_name],DATEPART(dd,[backup_start_date]);



As you mentioned normal full backups, above should be OK, but it could be tedious if you have quite a large no of databases. But i believe you can come up with query on same with help of sp_foreachdb. Also, make calculations based on the factor of retention period for the backups on that drive accordingly.


But the one which i have been using is this excellent article from Chad miller on Database Space Capacity Planning which with help of a Powershell and SQL script might do the trick by gathering the data say for next 15 days or a month to help you in deciding or predicting the Vol space for that particular drive ( in my case i have a dedicated backup drive, so this works good for me)


As said


  You can use the process described in the article to create a
  consolidated space forecasting report, which focuses on a "days
  remaining" metric. In addition, the use of PowerShell to collect data
  and load into a SQL table as demonstrated in this article, provides a
  solution you can easily adapt to many database administration problems