Just some advice with the pen since she’s so little: if you bought a pen that is meant for small animals (rabbits, ferrets etc. – these pens tend to be lighter and usually white in color) she’ll be fine in it even though she’s small, since the bars for these pens are closer together and she won’t be able to get her head or rest of her body through them.
If you bought one of the larger, heavier pens that are usually meant for dogs and that work well for adult bunnies (gold, black or silver colored usually and with bars further apart) she will most likely be able to at least get her head through the bars, if not the front half of her body. This has the potential of being dangerous, as she could hurt herself wedging herself through, or even get stuck.
My rabbit was only a few months old when I wanted to upgrade him to the pen, and I bought one of those standard gold dog pens from petco which I had to return when I realized that my bunny could stick half his body out of it. I ended up going with a bunny/ferret pen instead, which worked for him when he was small, but now is so light than he can move it around easily, which is annoying. I’ll probably switch back to a dog pen soon.
If you did buy a dog pen, you can just hold on to it until she’s a bit bigger, and only let her live in it once you’re sure it’s safe.