I implemented quotas this summer after getting fed up with clearing drive space 16 times in one year! We now have the following:

KS3 Students - 500Mb
KS4 Students - 1Gb
KS5 Students - 1.5Gb

Staff 4Gb
SLT 6Gb

Students studying specialist photography and media courses get a separate drive with their name on where they can store their photos or saved iMovie projects on. This is on a 1TB drive which is monitored by the course tutors who will also use this area to collect in and check students work as they go along. They will also check that they are using the space appropriately.