Hi Jane,
Do you mean a histiocytoma? If its not bothering her at all and doesn't seem to be causing her any discomfort, then you could leave it but if it was me, I would rather have removed while it was still small. Means less time under anaesthetic etc. Generally, anaesthetics these days are a lot safer and most young dogs recover fine from them. And like I say, if its removed while its still small, she wouldn't be under for too long. Vegas had to have two anaesthetics within about 1 month of each other and while thats not ideal, he was fine.