How big?

I was thinking of something in the 30' range, although a little larger at maturity is OK. No monsters, though.

Stewartia might take about 30 years to grow 30' tall, if you like that one try it. Tallest ones measured in Seattle 41'-42'+, after many years of growth.

Although De Wilde's reportedly had problems with winter injury in (I guess) their fields out off Meridian, tree is native to and grown in much colder places than Bellingham. Usual critical factor for it in PNW is lack of drenching rains in summer. Must be placed in soil that remains moist.

Downtown Everett has some nice ones growing out of small openings in pavement over what appears to be the most seemingly inhospitable Alderwood glacial till.

With a discarded cigarette butt mulch.

So, a soil remaining moist (as this location may, due to shading and density of the soil) could be the most important attribute - trumping other concerns including organic content.

Of course, the city may sometimes come along and water the openings - although I have seen no sign of this myself, the soil being dry on top when viewed later in summer.

Look for stock differentiated as Korean stewartia as that tends to be ornamentally superior, with more effective flowers and fall color, narrower growth lending itself better to planting sites with less abundant space - as with dogwoods and many other decorative trees you really cannot lop side branches back to control spread without spoiling the tree's appearance.

Or you could try something else entirely.

Here is a link that might be useful: 25 Trees that thrive in the maritime Northwest's Dry Summers.

I've grown Stewartia pseudocamellia var. koreana in Bellingham for 15 years with no hardiness or breakage problems. Wonderful fall color, peeling bark, flowers, and structure-- an all-year tree. It does grow slowly, a little over a foot a year, and needs a position where the soil doesn't bake in summer. I watered it a lot the first few years but now only after 2 or 3 weeks of drought. Mulching helps, as does allowing moss to take over on the ground beneath the tree. Really the only trouble I go to for this plant is rubbing lichen from its branches each year.

Even better for bark effects and fall color is Stewartia monadelpha, but the flowers aren't as showy. It seems to be a bit more tolerant of dryness.

The USDA puts Bellingham in Zone 8. Mulching makes a huge difference, when a soil is bare the sun bakes moisture out of it - you can actually see it rising up, as mist. When a root zone becomes shaded by branches or groundcover plants, this also produces a much better situation for roots of plants not liking hot and dry conditions. Sensitive types like rhododendrons may get root rot and die off even if a warm soil is kept damp.