The maximum icon size in Grid View that you can select with the slider is 128. But, there is a hidden option to set larger icon sizes. Open the Macro Panel (Window → Macro Panel) and run the following line of code:
Glyphs.defaults["glyphIconCollectionSize"] = 250
Instead of 250 you can pick any other number like 200 or 300. You need to close the font window and open the font again for the icon size to update.