To piggyback on Derek’s comments:
I think space concerns are important for a new church for two reasons. The lessor one is that the aesthetics set a tone that visitors pick up on. Dirty floors, stained (cracked vinyl flannel-back especially) tablecloths, and a selection of faded silk flowers say something about how the church regards itself, and thus any new members.
But there are bigger theological fish, too. Space tells us what – and who – are important.
I don’t get long-nave buildings for churches with a dominant preaching tradition (I’m thinking of a couple of monied Presbyterian churches) especially since they are almost always anachronistic. First Unitarian Church, Dallas, as an example, is very well set up for a “preaching service” (and would be a complete nightmare for a service of Communion, however distributed) and so the building fits its liturgy.
Also, yes, I think the clergy are important for a church, but there’s no excuse for having the architecture say (from the clergy) “I’m better than you are.” If there wasn’t a heritage of Thomas Potter and his chapel’s “square Quaker pew” we would have to invent one. I have some ideas (please comment on them) about what an idea first space might be for a small egalitarian Christian church. Perhaps it would be a useful way to recondition spaces that are too large for a current congregation, too. In the meantime, I’ll assemble a set of links for images that might get some of my points across.