Increase Icon/Text Size in Kali Linux

When you installed Guest Additions to your VBox Kali, there should be a bar at the bottom of your screen that shows up every time your cursor is there at the bottom (with options: File, Machine, View, Input, Devices, Help, Kali). I cannot take a screenshot of the bar but you should be able to find it.

Click on the View option, and you can choose full screen or seamless or whichever you like. But with high solution the icons will be small as you mentioned. It's okay because you can see at the bottom of the View list you have "Scale Factor", move your cursor on it and it will show different scaling such as 125% or 150%. I find 125% and 150% suit my screen the best.

If you are using Kali 2020, you can find it inside the Kali in the Setting-> Appearance -> Font. You can change Default Font size there as much as you wish there. Hope this helps.