Sentence:  Thirteen five-eighths-inch marbles (“<WSD>mibs</WSD>”) are set in a cross in the center. Each <WSD>mib</WSD> should be three inches from its neighboring <WSD>mib</WSD>. Each player shoots with a “shooter,” a marble that's one-half to three-quarter inches in diameter. 
Meaning of the word marked with <WSD> tags:  A marble (glass ball used in games), especially one used as a target.