Assuming your using VMware Fusion, since you posted in this forum, you could have clicked the Customize Settings button the Finish pane of the New Virtual Machine Assistant to make appropriate changes before installing the Guest OS.
After the Guest OS is installed you can change the amount of RAM and you can change the size of the virtual hard disk upwards (making it larger) however you cannot change the size of the virtual hard disk downwards (making it smaller) once it's partitioned/formatted or the OS has been installed. Although this could have been done had you selected the Customize Settings button and downsized it before starting the install of the Guest OS.
Since you didn't say what VMware Product and version you're using I'm not going to include any specific directions other then to say... Read the Documentation! 
If making it larger is what you want then that's easy however if smaller is what you want then you'll need to use appropriate utilities to image the existing virtual hard disk to a smaller virtual hard disk and then swap then out for each other. Or you can decrease the size of the virtual partition thus not allowing the disk to grow larger the the size you set for the virtual partition. This of course assumes non-preallocated disks and is not taking snapshots into consideration which can make the Virtual Machine's Package larger then the defines virtual hard disk size or virtual partition size.