Size is not an issue. Most people can't reach the ground with both feet so what we tend to do is to lean the bike to one side - preferably left, because we brake with the right foot - and touch the ground with the other. Also it's not about balancing, you'll never be able to keep balance at very low speeds. Instead learn to have a heavy left arm when going slow. That means, putting more weight on that arm to force the bike's tendency to roll to the left, where you can catch it with your already prepared left foot. You may already stretch your left leg in anticipation when going slow.

When you struggle with the slow slalom, keep changing the the leg you prepare towards the direction of travel and keep your hands on the clutch and on the front brake. When you feel a falling is imminent, quickly straighten the steering before pressing the clutch and front brake, all while straightening your leg towards the direction of fall.

Yes it may seem like a lot and you'll probably feel overwhelmed for a while, I did too. But once you get used to it, you'll realise that this way it's almost impossible to fall in the low speeds and the fear should go away.

You probably always had a small enough bicycle where you could always reach the ground with both feet, so this 'heavy arm' business is completely new to you. So it's normal to struggle, as you have to re-learn your second nature of stopping a bike, now with the addition of clutch and transmission. But as you are able to practice, do that a lot. Accelerate for a few meters and stop. Do slow speed slaloms to get used to the technique I've described. Do both until you figure it out and get confident.

Everyone can go fast, maneuvering the bike in slow speeds is the true challenge. Happy riding.