It's important to have the right size so they do they're job properly. I know it seems expensive, but once you're using them you'll realise a good cover is worth it's weight in gold! If it saves you from changing sheets, blankets, bubs clothes, your clothes etc etc a number of times a day, then it's worth every cent :) If it's too big it may gape around the legs, letting poo escape LOL.
We used the x-small sizes for ages. Also, if you're using less bulky nappies like flannellette, snibs are a really good way to go in the early days. I loved the eenees snibs as they are elasticated.
If you're looking at Motherease, go with the Rikki rather than the Airflow for a younger baby, you can get a better fit over the nappy because of the aplix. It's a great cover, they've saved me from many a disaster :D
I personally wouldn't use terry towelling on a newborn, I think it's too bulky. And we found the muslin wasn't absorbent enough at times. But everyone is different, and finds the best way of doing things for them. Maybe consider having some flannelette as well, you can always use them as spew rags if not as nappies (and you'll need plenty of them LOL).
