A couple questions...
Where do you have her litterbox in relation to where your bed is?
Kittens under the age of about four months will have accidents...mainly because they're still learning when to listen to their body telling them they need to "go". It would be a good idea to have a few litterboxes around the house, that way if she's not listened to her body's warning signs that she's gotta go, she's always close to a litterbox.
I would suggest having a box right by the bed at night.
For her age, you can easily just purchase the smaller litterboxes (about $5 each) to put around the house, and as she ages, you can steadily take one or two away here or there. You might want to purchase one that's just a plain box that's normal size to keep around in case you need a second one, but for now, I would get the smaller ones. I'm not sure of the exact measurments as far as how many per sq ft or anything, but at least one in each room would be good, and if you have two floors (or more) to your home, put one by the stairs both at the top and at the bottom.
If you put the box by the bed, and she still wets the bed, put her in her box to remind her it's there. She might forget. She'll get the hang of it, though! It's every cat's natural preference to go in a box.