The stock NSR150SP is fitted with a #152 main jet. Fitting a bigger main jet means increasing the identification number. So a #155 main jet will be bigger than a #152, as long as they're the same type and manufacture jet, and will flow more fuel, so will make the mixture richer.

The jets fitted to NSR150SP (and NSR250) are Keihin jets and the number stamped is relative to the hole size. ie a #152 jet is 1.525mm, a #155 jet is 1.550mm and so on.

If you're unsure as to what jet does what, and the actual name of the jets then I suggest you search around online and you'll find pretty much all the info you need.