Small trays are a lot easier for the owner but it wouldn't do any harm to try her with a big tray as well. You could always go back to the smaller one once she's being clean. Maybe she's just a bit stressed out with everything being so new to her and being away from her mum and litter mates. I'd try not to worry too much because I'm sure she'll be clean soon. I remember one of my cats once peed on top of my husband through the duvet when he was a tiny kitty (my husband was asleep at the time) but even Squeaky became housetrained eventually. We did have to confine him to the kitchen a bit though, with his tray, for a few weeks. I think that's another tip for housetraining a difficult cat, don't let it have the run of the house until it can be trusted not to pee or poo. Maybe confine her to the kitchen just until she's clean at times you can't be watching her (with a lovely cosy bed of course).