Simplicity has a pattern that was clearly designed for the Bitty Twins (the dolls in the illustrations look just like the retired Bitty Twins). All the Bittys have the same bodies so this pattern works well for Bitty Baby and the Twins. I don't have the pattern number handy, but you can find it on ebay-- search "clothes pattern 4 boy doll bitty twins" It contains patterns for a dress, overalls, shirt, PJs and jumper.

I love the Morrissey patterns. I have used their Bitty Baby, Hopscotch Hill and AG patterns and have been pleased with all of them.

You can make an educated guess about doll clothes pattern sizing if you look carefully at the dolls on the cover of the pattern. Is it a baby doll or a little girl doll? Is the doll chubbier than Bitty or skinnier? Some baby dolls are long and thin with small heads. The patterns designed for those dolls don't fit Bitty as well (you would definitely need to go up a size on those). The chubbier, snugglier looking dolls will be a better match. In general, it is better to err on the side of a little big-- it is easier to take something in than it is to make something larger. Sometimes the small print will say the pattern fits dolls such as and list some baby dolls by name.

Sewing for Bitty is such fun! Hope you find a pattern you're happy with!