Longer answer: What you see is the default Gnome Dash in the Application Overview. The size of the icons is automatically determined. There is no user exposed way to adjust the size of these icons: adjusting this would require inspecting and editing source code.
The size of the icons automatically is reduced when more icons are present on the dash. A workaround therefore could be to put more favorites on that dash. Icons would be smaller, but you would be starring at an entire column of icons in that scenario.
If the Dash annoys you and you would be fine not seeing it anymore ever, then there are Gnome Shell extensions that can hide the Dash.
Short answer: you can't.
Longer answer: What you see is the default Gnome Dash in the Application Overview. The size of the icons is automatically determined. There is no user exposed way to adjust the size of these icons: adjusting this would require inspecting and editing source code.
The size of the icons automatically is reduced when more icons are present on the dash. A workaround therefore could be to put more favorites on that dash. Icons would be smaller, but you would be starring at an entire column of icons in that scenario.
If the Dash annoys you and you would be fine not seeing it anymore ever, then there are Gnome Shell extensions that can hide the Dash.