I have it only for two years (from a very small 2G plant initialy) and judging by what I see so far, 4x4' will be more appropriate size to keep it. If you will be pruning it to 3x3' in a spring it will be 4x4' by July. Pruning to 2x2' will not work IMO since you'll be not able establish proper framework for the full plant in a such miniature dimensions.
If ground space is restricted you might want to try 4 (height) by 3 (width), which I think is doable, but that would need a careful training/pruning for several years as in my eyes it wants to be more wide than tall.
Good luck, it's a lovely plant.

Ego, thank you! I knew you would come to my rescue... :-)

I think I might just need to find another space for it, in order for the Little Lamb to really do well and show off its beauty...

You know, what really happened was, there was a sale in a local garden center, and they had all these 3 gallon hydrangea lovelies for $12.99! These are very healthy and big plants, so I bought a Pink Diamond, Little Lamb and a Kyushu!!! Now, where on earth do I plant them? Oh no, this impulse buying thing has to stop.........

Ostrich, the best placement of LL I've seen so far was in a middle of the lawn with no other companions.
That was indeed a cute white little lamb (about 3x4 or 4x5 from the distance) sleeping on the grass.
If I may suggest, I'd give PD more sun for the best and fast coloration and place Kuyshu in a such position where blooms will be seen more like a see-thru, not in a masse. Hard to explain and I don't have an appropriate picture to show, but basicaly you want Kuyshu's lacy flowers to dance in an air, not be just an undestinct white mass. Kuyshu's natural upright habit helps to make this fairly easy with selective pruning at the young age.
Not exactly, but something like this

Thank you, ego! I am so glad that you told me about the placement preference for Little Lamb - I think I might just have the perfect spot for it then! Thank you!

Regarding Kyushu and Pink Diamond - do you think that it would be odd if they are placed next to one another? I have a feeling that the Kyushu also deserves to be on its own too, to show off its foliage and its airy feel. I guess I really should place each of these beauties in their own position, so that I can show them off individually!

I have always loved your Kyushu tree, so I want to tell you again how much I enjoy seeing this pic! Thank you!

It may also depend on your zone and how quickly they grown in your area. In Ohio, you should be a little warmer than me (Madison, Wi), but still both zone 5. I have a Little Lamb that I planted in 2004. The rabbits helped keep it smaller last year but it is still only about 2' wide by about 1.5' tall. If it is anything like last year, when it blooms it will blend in with my perennials. For me, I don't think I would have a problem keeping it very small.

For me, Little Lamb is not as fast growing as Pink Diamond. And Kyushu is some place in the middle - and is growing as a multi-stem tree for me.

Hi - here we all are once more - we seem to be considering the same lovely H beauties. I planted LL H last year, same time my g'dau got her Princess Kyushu.

LL is so pretty - much like li'l Miss Kyushu but close to the ground - truly adorable. I planted her, just so. of steps/corner top of my ret. wall, where she sits beautifully, get's full sun 'til 3PM and seems to like her home. Another PW choice - lovely airy little one to consider. Mine is about 3'wdx2'ht now and loosely covered with blooms - just a bit sooner than Princess K!!!

Hi again - We had a big thunder storm the other night ... the little lambs were all asleep on the river rocks around them ('wish it was a grassy patch around - 'would have been a lovelier sight). In just 9 days the blooms that looked much like Miss Kyushu's airier form quickly filled up into the little lambs that they now are. It was quite a tangled web this LL created ... using some short folding wire fencing to hold the jute cord I wove around some of the old wood, I was able to prop the little guys back up ... I had forgotten how full LL's panicles get. I'll leave her unpruned and see how she'll look next to QF, next season.