Alarmingly, Toaster's alt mode doesn't seem to be the size of a normal household toaster at all, and is in fact around ten feet tall. While a questionable disguise, he's so rarely seen, it must be working pretty well for him.
Marvel Comics continuity
Toaster was among the Transformers rebuilt by the Ark when it awoke in 1984. He was given the alternate mode of a human bread-searing machine, colloquially known as a toaster.
Blaster, while hosting the letters page of the UK comic, told an interested letter-writer that he had a brother named Toaster who transformed into a toaster and whom he liked first thing in the morning. It's unclear whether this was always Toaster's name (coincidental alignment among Transformer's pre-existing names and Earthen altmodes is common) or if he changed his name to reflect his new Earthen altmode. (See; Goldbug)
Toaster seemed to all but vanish after the Ark's awakening, and went unseen for five years. It is possible that his naked power was so awesome and impossible to contain that the Ark simply forced him back into hibernation after realising the dervish of destruction he could become in his much-feared "giant toaster" altmode.
In 1989, the time-traveling Galvatron, caught in a series of time-loops and re-writes, remembered seeing himself triumph 20-some years earlier while he was Megatron. Toaster was among the vanquished foes at Galvatron's feet in this earlier iteration of the timeline. Toaster used the awesome power of his seldom-seem "giant toaster" form to attack Galvatron, but was defeated. Toaster did not appear in the "new" unfolding of events.
Items of note
Perhaps the most tantalizing thing about Toaster is that it's not clear if he's an Autobot or a Decepticon. All of his appearances take place at times when Autobots and Decepticons are together. Blaster's filial affection towards Toaster is limited to right after waking up, possibly squelched later in the day by the reality that they fight on opposite sides? Or because he just wants toast for breakfast as the joke implies... or he may have just been damaged for an extended period, such absences are hardly unknown. (The Autobot Sunstreaker was out of commission from 1984 to 1991.)