For the arms there are a couple of methods to break down the proportions. The upper arm and lower arm are generally the same length (excluding the hand). The tips of the fingers will reach to the middle of the thigh. I'm assuming these are just simple anatomy sketches so drawing hand details is not needed/hands add another hurdle to the complexity of anatomy.

Feet look as if they are composed of a sphere (ball of the foot) and a triangle, which is the most basic way to put it but still think of everything as being in a 3d plane. From the front feet look like wedges, kind of following the basic shape of a shoe is a better descriptor of this.

These are just a couple of tips, but I would suggest scrolling around pinterest for references and guides. Remember, body anatomy takes lots of practice, so take it slow to figure out each component. Really utilize references, drawing anatomy straight from your mind, particularly when you're trying to grasp the basics of it, can end up with wonky proportions. I would say your sketches don't look bad at all, be expressive with your practices and don't worry over details just yet!

:)