First softbox: what size?

I'm thinking about making the plunge into studio lighting with a Profoto D1 unit. I don't want to overwhelm myself with "too much change", so I'm going to start with just one unit. I have some basic umbrellas and I'm sure the Profoto umbrellas are a notch (or more) better, but I want to dive in feet-first and get a softbox to start. Question is: what size? I'm thinking a 2'x3', as it somewhat simulates the size/shape of a window, or I can turn it sideways and have a broader source. (Certainly I can and will buy more softboxes in time to get some size variety...) What size would others recommend?

I'd go with something you can't easily do with speedlites, say a large (4-5') octabox.

The 2x3 is a good softbox size. It's got a nice spread. I use these in a studio environment. It does depend on the size of your studio, also. The bigger the studio, the bigger box you can accommodate. Of course, the bigger boxes cost a lot more. Paul Buff Inc. sells excellent softboxes and accessories.

I took the plunge a week or so ago, and picked up a used Profoto D1 500/500ws Air kit, along with a new 2'x3' softbox with grid. I added a 1/4CTO gel to the front baffle (clipped to the baffle, and further restrained by the grid). I've put the two included umbrellas in the junkpile (they're tiny) and dug out my largest (a 46") as a modifier for light #2. I did some selfies for a few days, then got pressed into doing some portraits for a friend Monday night and really enjoyed the results. However, the big "deadline" was a batch of portraits at a "board meeting/dinner" for a non-profit last night. Out of 200+ shots, I think I had one shot where a D1 didn't fire, but plenty where my background light (a Speedlite 580EX II on a pocket wizard) didn't fire. More importantly, the light is absolutely consistent, and beautiful to boot.

Up next, a 3' Octa for the second light, then perhaps a 1'x4' stripbox for some creative options. Thanks for the guidance on the 2'x3' box!