610 entries with a sense WordNet labels colloquialism. Register matters more than meaning when you are choosing between two words that mean the same thing. Membership is WordNet's own register label: every entry here has at least one sense marked "colloquialism".
Commonest, longest and average syllables are measured across the 600 words carried on this page, ordered by how often each appears in everyday English.
610 entries match this rule. This page carries the first 600 of them, ranked by commonness, so the list stays readable and the page stays small. The rest are still in the dictionary — search for one, or browse the letter it starts with.
Definitions, subject classification and semantic relations come from WordNet. Syllable counts come from the CMU Pronouncing Dictionary where the word is recorded in it. Full sources and licences.