I feel your pain! As another small-framed woman, my shoulders are the widest part of my body, which makes fitting very interesting, indeed. I am going to send you to two sites, one that shows some basic tunic shapes, and the other for tips on taking your own measurements.
A google search for "rubakha" turned up a good handful of images that give clues to the garment's construction, but since I'm not especially familiar with Russian garb and I can't read anything written with cyrillic characters I had to do some guess work.
Following the basic pattern for the Type 5 tunic will allow you to have shoulder seams at your actual shoulder, while still allowing you to wiggle into your garment without too much trouble. Type 3 appears to be closer to a rubakha's actual construction, but you'll probably end up with a shoulder seam somewhere on your bicep.
This second link takes you to a 16th century costuming page, but I think it'll still be useful to you.
If you are a novice seamstress and all this freaks you out, ask around to find somebody willing to teach you the basics of geometric construction. It is economical (yay for fabric conservation!) and period-appropriate for a wide span of time and geography within the SCA.
If you have more specific questions, feel free to come back and ask.