Unless you use reverse incremental mode which roll each day into the full backup, you will need enough space to keep 2 fulls on disk. Doesn't matter if you use synthetic full or active full, they both need enough space to create the backup on disk.
There is also forever incremental mode like foggy said where it creates the first active full and then a long chain of incremental after that. While it doesn't use as much space as forward incramental, you have to read up on this mode as I believe restore speeds and risk of losing your chain to a bad backup are greater.
My synthetic fulls take longer than my active, but the advantage is no open VM snaps with synthetic full's putting load on your VM's like with active fulls.All the synthetic full processing is on the backup storage and proxies. I think the key to synthetic fulls is testing your backups periodically to make sure they are good. https://helpcenter.veeam.com/backup/vsp ... thods.html