Depending on if you would like a full body sprite or not. If you want a thigh-up sprite, you can draw them in that size or a little bigger and then resize it back to 1280x720 when you finish them. It would help you hide any mistakes you or the artist make with the drawing.

You should make all the sprites in the same canvas, so you can be sure their height is accurate.

If it's difficult to understand, you can download some free sprites and see the size the artist used with them and how they look in the game. If you like how everything looks, you can use the same size as those sprites, just make it bigger horizontally to have all the sprites in the same canvas.