You can select your preferred icon size from the right side dropdown menu, as shown below.
Currently, you can filter sizes between 16 and 512 pixels. You can preview how the icons will look like, as they automatically resize when you choose a different size.
If you have found an icon you like, you can also select the size from the horizontal bar just above the icon. You can also preview how the icon will look like in different sizes and choose the best one for you.