As your post needs moving to the area for ESXi, I have reported it to the moderators.
First thing to do is find out which datastore the VM resides in - the Summary tab of the VM will show that. Then go to the host in the tree on the left and use the tabs to find out more about the datastore.
The solutions are self-explanatory - either the datastore needs to be bigger, or you'll need to move some files out of it to free up space.