Hi I have to cut the bigger piece to the size of smaller piece,

Score and snap if you're feeling brave. Grinding it with a fine wheel if your not.

There is a technique for cutting glass tubing where you score around the tube using a diamond point or glass scoring tool, and then you use an electrically heated piece of nichrome wire to to heat the area you just scored. It causes a crack to begin as a result of expansion of the glass and makes a neat cut.