Can I use a D size title block?

Yes, you should be able to.  Do you not have access to a D-sized sheet format?

You can use whatever size title block you want.  As long as your company drafting standards allow

I only use D size. B size text minimum is (I think) .12 inches--way too big, way too crowded.

D size with .16 text reduces beautifully to B size.