I think that `boot` you are talking about means your ubuntu installed partition's size. For that, boot from live CD or USB into ubuntu. Then, open an application named gparted to resize partitions. (This might break your bootloader and might requite to fix it)
A better idea is to make a new partition and then move your home directory there so that you will have additional space and Ubuntu partition reserved for Ubuntu.
Simply follows step below,
- Boot from live CD or USB
- Run gparted to shrink or delete current windows partition. (Make sure you will have backup for files from Windows, though shrinking a partition will not delete any of your files, but backup is always a good idea)
- Create a new ext4 partition in the empty space.
- And follow this guide to move your home directory there.
Important: Do not forget to old /dev/sda3 in /etc/fstab. Multiple declaration of volume will crash your ubuntu. Cheer!