I'm very familiar with the phrase [in Australia]. For example: If someone told me that there were sixty people coming to a wedding I could reply 'That's a good even number'. Now I'm really thinking about this...would I reply the same way if the number was 'sixty five'? Probably not. As 'rspreng' suggests it is akin to ' a nice, round number' or simply 'a good/suitable number'. It's one of those expressions that people around me use without thinking about where it orginated.