Just read a discussion on another midwifery list about previa (haven't dealt with it myself). While some HCPs believe that placenta should be at least 2 or 2.5cm from the os to be safe, others said that their sole criterion for vag/homebirth is the amount of bleeding. That is, they are go for homebirth *unless* mom has anything more than normal bloody show during final weeks and during labor itself.
And as for your baby's size...well, yes, more protein could help baby grow larger. However, it might be that with a marginal placenta, you/your body/your baby have a positive intention in making a smaller babe....could be easier on that placenta maybe?
Aside from diet, I add that stress can well impact baby's growth. Are you experiencing a ton of stress over this previa issue? I mean, anyone might! Just to say that along with diet, attention to de-stressing/relaxation and positive visualization might also help w/baby's growth if stress is indeed an issue for you with this pregnancy/previa.
all the best with this....