My smallest kit is actually a moleskine pencil case with an elastic band on the back to hook onto a journal/sketchbook. I picked it up in Halifax last fall and it fits my Platinum Preppy 0.03 fountain pen with red ink, a Faber-Castell poppy red pencil crayon, a pencil sharpener, Pentel mini waterbrush and small Altoids tins filled with Daniel Smith hansa yellow light, crinacridone rose red and indanthrone blue watercolours.
My medium kit is a waxed Peg & Awl maple red Sendak Mini filled with Jane Davenport watercolours & pen filled with D'Atramentis permanent poppy red ink, larger waterbrush, white Posca acrylic marker, white Signa Uni-Ball archival pen, small watercolour brushes, a collabpsible water dish, mechanical pencil, erasor, sharpener and a Faber-Castell red pencil crayon.
My largest kit is the 6x8 Judson's Plein Air ThumBox (they make them a little differently now) and it has amazing storage. I use it as an easel and have now filled it with watercolour brushes, water dish and White Nights watercolours since I can no longer use acrylics due to environmental allergies in my home. I've had it for fifteen years still absolutely love it! Though I use all three kits all of the time, it is definitely my go-to in my studio.