In general, I would recommend getting a size 6 watercolour brush if you're working on an A5 sketchbook or smaller. For larger sketchbooks like A3, it's would be better to get a size 8 or 10 because they have a bigger wash area.

Note that Escoda's size 10 travel watercolour brushes will not fit into those small metal 12-pan watercolour boxes. Check out all the watercolour brushes I've featured at https://www.parkablogs.com/tags/watercolour-brushes

For more sketching tips, visit this Youtube playlist and this page.