Lizzy, I make cotton napkins all the time, and like them to be at least 18" (about .5 m) square. However, if you're using 45" fabric (about 1.3 m?), you can make them a little larger, to take advantage of the width. It's hard to have a napkin that's too large, IMO.
Rolled hem on the serger works fine, but I also like to simply make a twice-turned 1/4" hem, too.
Have fun; it's nice to have a "wardrobe" of table linens, especially holiday ones. Karen Maslowski in Cincinnati