Generally you can set only three sizes of icons –Large icons, Medium Icons, Classics Icons
Right click on Desktop > View > and choose icon size
But if you want choose more size of icons Go to Desktop, Hold down Ctrl key and use mouse Scroll
For Increase Icon Size – Ctrl + Scroll Up (Big size)
For Decrease Icon Size – Ctrl + Scroll Down (small Size)
If you want to set old icon size again, Right click on Desktop > View > and choose icon size> Medium Icons.
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