Can I increase icon size in task switcher?

It's possible to increase the thumbnail size in the ALT-Tab window without a 3rd party tool.

See instructions here. They give a registry file that adds several entries to the Registry that control the behavior of the ALT-Tab windows and thumbnails.

I guess you're interested in changing the MaxThumbSizePx entry.

Play a bit with the settings to find what you like. No need to restart, they're picked up on-the-fly.

If you want to change the number of rows or columns displayed in the ALT-Tab window, see the settings CoolSwitchColumns and CoolSwitchRows, explained here by Microsoft. Restart needed when changing these 2 values.

Be very cautious of course when changing the Registry !!!!

Try vistaswitcher. It gives large thumbnails and a bunch of other functions. Screenshots are on the linked page.