Have you tried measuring it?

Yeah, tried that, but I'm not sure where to measure the stem from...is it end to end or from the middle of the base bar to the middle of the steer tube? Also, I don't have a good way to measure the circumference of the base bar circumference in mm. Any suggestions?

stems are measured middle to middle. Often the length will be stamped somewhere on the stem, but not always. Alternatively you can just take the stem off and go to the local bike shop and tell them you want something that is 'x' mm shorter / longer.

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The stock base bar should be 26.0. (Vision)