Lavandula angustifolia, sometimes known as English lavender, is the only z5 hardy lavender, and then only in select hardy cultivars like 'Hidcote.' The rest are only reliable in z6, and many of the Provencal and Spanish species and hybrids are z8. Don't worry, though, Hidcote is a great cultivar and very fragrant.

Lavender spreads fairly quickly if it's happy; in the ground it'll eventually get to around two feet in diameter, but you can always prune out new suckering growth, and LOTV is pretty easy to dig out if it gets too close -- a greater concern is, if you're siting it next to a semi-shade plant, does the location get enough sun to keep the lavender happy?

Jim