ok, we seem to have gone from a 3mb log to a 14GB one overnight. Just change the growth factor but otherwise leave the logs alone. HAve you changed the backup frequency to be more often?

After a week run this script whilst connected to the database to get the largest log backup and decide a size from there

declare @dbname sysname

set @dbname = db_name()

print @dbname

select backup_start_date,backup_finish_date,backup_size/1048576 as 'size in MB'

from msdb..backupset

where database_name = @dbname and type = 'L'