In my area, 6-8' tall and wide is not at all uncommon for this shrub. The cultivar name 'Nana' should be taken with a grain of salt, as it is not all that much smaller when mature than the straight Arctic willow. And yes, they handle pruning well :-)

To maintain the size of ours, we just whack it back to the ground every 3 years or so in late winter. Ours remains about 4'-5'.

The funny thing is that several large reputable nurseries advertise the height and spread at 4'.

Bailies Nursery says 3'-4' mature height!
McKay Nusery says 4' mature height and spread!

Its just crazy that I planted them last spring and they were maybe 2ft tall. Now they are 5ft tall, caliper of stems went from pencil thick to 1" thick.

Willows are notorious for fast growth, but with that comes weak wood. So it's not only a good idea to keep them pruned so that they fit where you placed them in your design, it may also be required in fact, if the wood splits in an ice storm, for example, or sustains other damage. They will quickly recover.