Boulder marbles are approximately 1" or 25mm in diameter.
This is the size that we use to play marble bowling and many people use this size marble as their shooter. They are close to the same size as a quarter in diameter. They usually range from 23 - 27mm in diameter. All sizes are approximations and can vary from one marble to the next. Due to the way that they are made, machine made marbles have creases in their surface, which may be noticeable in this size.
