Grandiloquent is usually considered an autological word.

It means pompous, lofty or extravagant in language, style or manner, especially in a way that is intended to impress or attract admiration.

The word’s origin is late 16th century. It stems from the Latin grandiloquus, meaning ‘grand-speaking’.  The association with eloquent has altered the ending in English.

Magniloquent is considered a synonym of grandiloquent. And they rhyme.