When you change your realm size you also have to type /reset YES while it your realm.  Then your size change will take effect.

i tried sign in then i cant find the size button thing pls help me ths :)

Log in to your account at http://www.womrealms.com and then click the edit button, scroll down and you will see a box showing your allowed voxels, a preview of your realm(s) and the option to change the number of voxels in your realm, therefore changing the size. Then go onto World of Minecraft: Realms, enter your realm (the one you want to change) and type /reset YES. If you don't want to lose all of your realm you should /copy all of your world and paste it after the change, (remember to copy from the lowest corner to the highest opposite corner to ensure you don't just copy the base of your creations), you might want to use /copy world for ease.