Disk Utility hangs prior to resizing partition
Yeah I do encounter this problem sometimes also, but I don't know why is it happening. For some verifying and repairing solves the problem, but for those that doesn't get solved, the command line tool diskutil often does the job.

You can just use

diskutil resizeVolume $1 $2


where


$1 is the Mount Point or Disk Identifier or Device Node of the disk you are trying to resize (obtained with diskutil list).
$2 is the size in decimal with suffix of B, S(512 byte blocks), K, M, G, T, P, or percentage of the current size(%).


e.g. diskutil resizeVolume disk0s6 100G will resize the volume disk0s6 to 100 GB.