Here is, I think, a partial answer. I believe I can show that as long as a countably closed forcing adds a new omega1-sequence, the continuum is collapsed below the size of the poset. I am not sure if you can do better.
Prop. Let mathbbP be a countably closed notion of forcing such that Vdashdotf:omega1rightarrowON,dotfnotinV. Then, if G is mathbbP-generic over V we will have V[G]vDash2omegaleq|mathbbP|.
Pf: It's enough to show that in V[G], |mathcalP(omega)capV|leq|mathbbP| (because mathbbP is countably closed). Note that for each pinmathbbP there is some alpha<omega1 such that p doesn't decide dotf(alpha) (otherwise f could be defined in V); let alpha(p) denote the least such alpha. Let beta0(p)<beta1(p) be the least ordinals beta such that there's qleqp for which qVdashdotf(alpha(p))=beta.
Fix in V a well-ordering prec of mathbbP. Now, working in V[G], we associate to each qinmathbbP an xqsubseteqomega as follows. Inductively define a descending sequence of conditions q0geqq1ldotsgeqqnldots by q0=q, qn+1 is the prec-least member of G below qn which decides dotf(alpha(qn)). Let xq=ninomega|f(alpha(qn))=beta0(qn) .
To finish we just have to show that for each xinmathcalP(omega) that the set Dx=rinmathbbP|rVdash(existsqindotG)x=dotxq is dense. Let pinmathbbP be a fixed condition. Inductively define p0geqp1geqldotspngeq. Set p0=p. If ninx set pn+1 to be the prec-least member of mathbbP with pn+1leqpn and pn+1Vdashdotf(alpha(pn))=beta0(pn); if nnotinx then do the same thing but have pn+1Vdashdotf(alpha(pn))=beta1(pn). Then let r be below all the pn. Then rinDx, with p as our witnessing q.
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