Here are all the cheats I know for Masters of Teräs Käsi:

Level select:
Select Chewbacca as a character, set the difficulty level to "Standard", "Player Change" to "No", and "Continue" to "No". Then, beat the game in practice or arcade mode. Now level select will become available in versus mode.

Play as a Stormtrooper:
Select Han Solo as a character, set the difficulty level to "Standard", "Player Change" to "No", and "Continue" to "No". Then, beat the game in arcade mode.

Play as Slave Leia:
Select Princess Leia as a character, set the difficulty level to "Jedi", "Player Change" to "No", and "Continue" to "No". Then, beat the game in arcade mode.

Play as Darth Vader:
Select Luke Skywalker as a character, set the difficulty level to "Standard", "Player Change" to "No", and "Continue" to "No". Then, beat the game in arcade mode. Note: The hidden Mara Jade character will appear instead of a mirror match when playing as Darth Vader.

Play as Mara Jade:
Set the difficulty level to "Jedi", "Player Change" to "No", and "Continue" to "No". Then, select team mode and hold L1 + L2 + R1 before the first match starts. Now defeat the side designated as the Empire to select Mara Jade as a playable character.

Play as Jodo Kast:
Set "Player Change" to "No", and "Continue" to "No". Then, select survival mode and defeat at least seven characters.

Full screen:
After choosing a character, hold Select + L1+ R2 and keep the buttons held until the match begins.

Big head mode:
Select versus or practice mode. Then after choosing a character, hold Select and keep the button held until the match begins.

Super deformed mode:
Select versus or practice mode. Then after choosing a character, hold Select + Up + X and keep the buttons held until the match begins.

Small fighters:
Select versus or practice mode. Then after choosing a character, hold Select + Up + X + R2 and keep the buttons held until the match begins.

Big head, feet, hands, legs:
Select versus or practice mode. Then after choosing a character, hold Down + X + Select and keep the button held until the match begins.

Alternate costumes:
At the character selection screen, press L1.

Control credits:
During the credits sequence, press Up or Down to reverse or speed up the scrolling.