W(2:end-1,2:end-1)=U;
using the fact size(W you already created is size(U)+2 and you wanted it centered. If it weren't centered, then the generic expression is written based on the size of U instead of W...
r1=2; c1=2 % row column starting points in target
[nr,nc]=size(U);
r2=r1+nr-1: c2=c1+nc-1; % end positions in W
W(r1:r2,c1:c2)=U;
The rule is the size() of the LHS addressing expression has to match that of the RHS.
