What a great pattern! I love it.
I have made garments over vintage patterns in the past. The instructions and sizing are both weird.
The pattern may have a size chart on it. If so, you'll see that women were a lot smaller in the 40s.
What is now called a "12" was a "16." So you're right; it is
smaller. Patterns pre 1960 or so vary by about 2 sizes and patterns in the 60s and 70s vary by about one size. Sometime in the 80s they switched to the sizing system we have now.
Anyway, the real trick is to measure the pattern itself at the bust, waist, hips, and everywhere else that it could be important. Subtract out the seam allowances and then you'll know if it'll fit or not. This looks pretty tailored, so there likely isn't too much design ease, but measurements should range from 1 to 6 inches larger than your actual size.
Starting with a pattern the right size is nice, but it's not necessary. If the pattern's not right, you can use it to draft a new pattern in your right size. If it's close to your size, you can add and subtract at the appropriate places pretty easily. In fact I do this with every pattern. Like ready-made, the pre-printed sizes don't match me, and I figure that making your own clothes gives you a wonderful opportunity to make the darn thing fit right.
My first advice to friends about sewing is to throw out the pattern guide and my next is to not cut the pattern out on their lines--use your lines.
Okay, enough santimonious pontification. Stopping now.
Oh, no I'm not, actually. I'm still going!
I just wanted to say that jackets are difficult and I'd make sure I was familiar with their construction in general before diving into a vintage one. Okay, done now!