changeelement — otkritka

const circle = document.getElementById('circle');
const sizeSelector = document.getElementById('size-selector');
const colorSelector = document.getElementById('color-selector');

sizeSelector.addEventListener('change', () => {
    if (sizeSelector.value === '1') {
        circle.classList.add('size50');
    } else {
        circle.classList.remove('size50');
    }
});
sizeSelector.addEventListener('change', () => {
    if (sizeSelector.value === '2') {
        circle.classList.add('size100');
    } else {
        circle.classList.remove('size100');
    }
});
sizeSelector.addEventListener('change', () => {
    if (sizeSelector.value === '3') {
        circle.classList.add('size150');
    } else {
        circle.classList.remove('size150');
    }
});
sizeSelector.addEventListener('change', () => {
    if (sizeSelector.value === '4') {
        circle.classList.add('size200');
    } else {
        circle.classList.remove('size200');
    }
});
sizeSelector.addEventListener('change', () => {
    if (sizeSelector.value === '5') {
        circle.classList.add('size250');
    } else {
        circle.classList.remove('size250');
    }
});

colorSelector.addEventListener('change', () => {
    circle.style.backgroundColor = colorSelector.value;
});

Results:

Задайте радиус круга: Задайте цвет круга: