In this work, using the maskless lithography technique, we have fabricated a series of square Ag/SiN/Ag magnetic absorbers with different side lengths. Multiple absorption peaks occur and are analyzed with the help of their field distribution simulation. The absorption peaks coming from the first and third magnetic resonance modes can be arbitrarily modulated by changing the side lengths. The magnetic absorbers exhibit strong absorption, polarization selectivity and arbitrary adjustability. Absorption peaks dominated by the first magnetic resonance modes possess high polarization selectivity, whereas absorption peaks dominated by the phonons of SiN are polarization-insensitive. Such magnetic metamaterials would advance laser techniques and optical device applications.