Hi,
This guide shows you how to install Windows in Mac using Boot Camp , it has a step to set the partition size for the Windows installation in Boot Camp Assistantwhich is helpful for you.
(the guide is for Windows 7, but similiar to Windows 8\8.1)
https://support.microsoft.com/kb/2647609?wa=wsignin1.0
The following guide shows how to remove Windows from Mac
Boot Camp: Remove Windows from your Mac
http://support.apple.com/kb/PH17861?viewlocale=en_US
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