To put things in obvious terms, ask her.
There's going to be a date at which she wants to consider herself as female, so respect that.
One day, he'll go into the gents toilets, and the next she'll go to the ladies.
As it's a small company, it shouldn't be too hard to get this simple idea across to people. Yes, there's going to be some transition for people getting their head around a change in gender, but the important thing is that it's the same person. Same experience, same skills, same knowledge. Nothing about his/her working practices should change apart from dress code and perhaps an email address (which happens anyway when women get married, so you have a process for that).
But ask. She's probably thought about this for a long time and has looked for advice on this and has her own idea on what will help her and you (as a company/colleagues) feel better about this transition.