The term "MBR" is an abbreviation for "Mater Boot Record" and has a specific technical meaning. The MBR is located on the first sector of your hard drive and is a fixed 512KB in size; it contains your partition table and any bootloader that you have installed. The MBR cannot be "resized" so your question doesn't make a lot of sense as posed.
From the error message in your post, it appears what you are trying to say is that you have set up a separate /boot partition and it is apparently filled and as a result, your system will not allow you to install a new kernel. You can try resizing your /boot partition and make it bigger but back up before attempting any partition resize operation as there is some risk of data loss. A good free tool for working with linux partitions is gparted:
http://gparted.sourceforge.net/
Get the livecd version and see if you can resize the /boot partition to give you a little more space that you need.