I get this every once in awhile. It happened just this morning as a matter of fact.
Open display properties...appearance. Change the font size in the dropdown menu to large and then back to normal. Then hit apply. That will get the icons back to normal size. If they still look like crap then open task manager (control/alt/delete) find explorer.exe.
click on it and end the process. Then click on FILE..Run.. then type in explorer.exe..run. Should be all fixed.