I like the 30x40. Unless you plan on adding curtains in the future, then I would lean towards the 30x30
Unfortunately we won't be able to hang curtains with the way the blinds are mounted. It feels a bit unfinished to me but DH really likes the clean look of no curtains (and doesn't voice many decor opinions) so I'll probably keep it that way.
I agree with mrsukyankee - 36+ square in bedroom and rectangle on dining room.
In the bedroom I think the rectangle shape matches the window shapes too much. The square looks more modern.
In the dining room I think a square that is large enough would be hidden too much behind the head of the table (if you never have a chair there then maybe disregard).
I agree with jamaicam. For artwork on walls, I’m almost always “bigger is better.” I like spaces to be filled, but hate hanging collections or collages.
Update: ordered the large (36x48) rectangle for the dining room! Even though it’s a giclee canvas it is definitely the most expensive piece of art we have ever purchased but I love it so much and DH actually doesn’t mind it. Plus it’s from a local artist