The following query will help you get the size of all the full backups (if available) in the instance.
select Database_Name,
backup_start_date BackupStartDate ,
convert(numeric(10,2),(compressed_backup_size/1000000)) 'BackupSizeIn MB',
bmf.physical_device_name
from msdb..backupset bs inner join
msdb..backupmediafamily bmf
on bs.media_set_id = bmf.media_set_id
where type = 'D'
and backup_start_date = (select max(backup_start_date)
from msdb..backupset bs1
where bs1.type = 'D'
and bs1.database_name = bs.database_name)
order by convert(numeric(10,2),(compressed_backup_size/1000000)) desc