I think the main thing is to think 'what can you reasonably do about this?'
you could try propping yourself up with pillows so you can't roll over, but short of actually using guy ropes, you can't stop it happening.
women have been having babies for thousands of years without people telling them not to sleep on their back, and yet the human race is doing ok, so I really wouldn't worry about it.
Honest truth? Stress is much more likely to affect the growth of your baby, so focus on relaxing and trying not to worry too much. I know that's easy to say, but nice warm baths with lovely oils, hot milky drinks before bed, and all that stuff.
also, don't worry too much about measuring small. again, it's not something you can do much about, so try not to worry. I know lots of women who have measured small and had extra scans and loads of fuss, and had big bouncing babies at the end of it! depends on your physical frame, how good your stomach muscles are, how you're arranged inside, etc etc.
Measuring the bump is not an exact science.
Hope this helps a little