I got this one as my first and I find it very comfortable even when riding a bike! Plus it looks gorgeous and the glass allows for temperature play. Definitely a winner in my opinion! I have a review on it which you can find on my profile. Note that I had never put anything up my bum before trying this plug except for a finger once (only one finger, no more!)
If she wants to wear it for long periods then any of the ones mentioned should be fine. I'm not sure about the Lovehoney sensual glass rosebud plug though. I've just tested that and whilst I found it great for all day wearing it is a little on the heavy/large side for a beginner....in my opinion xx
I would go with any of the jewelled metal ones above and would choose medium size it's all to easy to get one that's too small that can pop out whilst having sex and then end up buying a bigger one after a short time you'll be surprised just how easily larger ones can be accomodated . If she's going to wear for a while(out and about ) try round the house first, and a oil based lube helps in that case as it will not dry out.