Enlarge Mac OSX disk
The following seems to be the solution. I haven't tried it myself though.

Copied from here:


Go into Disk Utility and click on your main hard-drive that say either 256 GB, 517 GB, or 728GB.
Up the top, Click Partition.
Then down the bottom of Disk Utility, Click the + sign.
Now, select the new startup disk by clicking on it and then name it "Macintosh HD (Lion Installed)" or you can name it what ever you want.
Now size the new startup disk to the size you wanted your other startup disk with Lion installed on it to be and click apply.
After the process is done, Restart your mac and while it is starting up, hold down options.
You will see a grey screen for a second, but don't worry, it is normal.
Now, all your startup disks that you have will appear on the screen. Click Recovery HD.
You will see a grey screen for a second, but don't worry, it is normal.
Now, you will see the option to go into disk utility. Click on it and then click to continue.
When disk utility opens, click on your new startup disk on the left side of disk utility, now click on restore, now you will see 2 rectangular boxes that say source and destination.
Drag your startup disk that has Lion installed into the source box, then drag the new startup disk that you made 3 minutes ago into the destination box and click restore.
Once the progress is done, you can delete the old startup disk that had Lion install on it by going into your main startup disk that says 256 GB, 517 GB, or 728 GB and then clicking on Partition and then selecting that particular startup disk and then hitting delete on your keyboard.
(Last Step) Now, you are ready to restart you mac by clicking on the apple sign in the top left corner of the screen and clicking startup disk, now you will get the option of choosing which startup disk to startup your computer with. You just choose "Macintosh HD (Lion Installed)" or whatever your new startup disk is called and then click Restart.


Your Computer will now restart. Now all of your files that were on your old startup disk with Lion installed will be on on this brand new fresh startup disk on your beautiful Mac.