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.
Using white gel pens is an easy way to introduce white back into watercolour sketches. It's definitely more convenient than using masking fluid and for outdoor sketches, you don't have to bring masking fluid along. Click for a zoomed in image