Have you looked at the functions available for graphs (see the 'Object Functions' section) in MATLAB? It appears that conncomp will do exactly what you're looking for.
G = graph(); % put your graph here
[bins,binsizes] = conncomp(G); % Find the connected components in G and find the number
% of nodes in each connected component
% binsizes = number of nodes in each connected component
% bins = vector explaining which bin each node goes into
