B is an RGB image, so size(B) is a 3 element array. When you pass in a 3 column vector or matrix into imresize, it expects it to be a colormap, not a size, and if it's a colormap it must be doubles in the range 0-1, not integers in the range of 0-thousands or whatever your size is.
You need to get the rows and columns by themselves. NEVER use size() like that on an image you read in with imread(). Why not? See this: http://blogs.mathworks.com/steve/2011/03/22/too-much-information-about-the-size-function/
The fix
[rowsB, columnsB, numberOfColorChannelsB] = size(B);
% Now resize A and overwrite the old B
B = imresize(A, [rows, columns]);
