The slow jet choice depends on your specific bike and your elevation. With a stock pipe near sea level, I'd stick with a 25. With a free-flowing muffler, try a 30. Whatever your bike needs to start quickly and idle smoothly.
Another way to look at it is if the pilot mixture screw needs to be backed out more that two full turns, your slow jet is too small.
Remove the slow jet and main jet from your carb and then hit
www.jetsrus.com. Compare the jets from your Mikuni to the jets in the pictures, and ensure that the measurements match. Once you've found a match, you'll learn what the numerical designation is for your jets, and which point you can shop at
www.jetsrus.com or eBay.