0201

0201 refers to a part size common to resistors, capacitors, diodes, and other passive components.  An 0201 part is 20 mils (0.020 inches) long and 10 mils (0.010 inches) wide.  These parts are difficult to see without magnification equipment and hand soldering of these parts is all but impossible.

0402

0402 refers to a part size common to resistors, capacitors, diodes, and other passive components.  An 0402 part is 40 mils (0.040 inches) long and 20 mils (0.020 inches) wide.

These parts are very difficult to solder by hand without the aid of magnification equipment and a very small tipped soldering iron.

0603

0603 refers to a part size common to resistors, capacitors, diodes, and other passive components.  An 0603 part is 60 mils (0.060 inches) long and 30 mils (0.030 inches) wide.  These parts can be seen without magnification and can be soldered by hand with some difficulty.

0805

0805 refers to a part size common to resistors, capacitors, diodes, and other passive components.  An 0805 part is 80 mils (0.080 inches) long and 50 mils (0.050 inches) wide.  These parts can be hand-soldered without too much difficulty.