The suit does exactly what the name says it does; it shrinks itself and its pilot down to any size the pilot may wish. It has one main drawback in that it can only shrink, and therefor the pilot must be able to wish themselves back to normal size by other means.
The suit is white with gray trimmings, and is vaguely humanoid in shape with arms, legs, hands, and a built in head from where the suit is controlled. There are also two lights on the shoulders of the suit.
When the suit transforms into its sub form, it maintains its color scheme, circular head window, and two lights, although the control head is moved to the front of the suit and the legs become two thrusters.
The Shrinky Suit was first used in "Tiny Timmy!" when Timmy used it at first to study the microbiological world for a school report. It has dozens of "gizmos and gadgets" for self defense against micro-organisms. It can also change from a "suit like" structure to a submarine like structure. When Cosmo and Wanda are swallowed accidentally by Vicky, Timmy changes the suit into the submarine form and enters her body, where he observes her heart (a literal black hole) and the tiny men inside her brain that run things.
After disrupting some of Vicky's brain functions, Timmy is chased in the Shrinky Sub by antibodies. Timmy reaches Vicky's voice box and a megaphone appears from the top of the sub to allow Timmy to speak into the box and wish for himself to be returned to normal. Timmy and the Shrinky Sub are poofed outside of Vicky's body moments before the antibodies fired on the sub en masse, although the sub did not appear outside of Vicky's body with Timmy, nor was it used again for the rest of the episode.
The Shrinky Suit's last appearance was in "Escape From Unwish Island" where it was revealed that Cosmo had broken it while playing with it along with Timmy's Time Scooter. It has not reappeared since then.