Indeed. Compound modifiers are one of the only 'rules' of hyphens.I think it needs a hyphen whether it's formal or informal. Lord knows there aren't many rules regarding hyphens, but one of the best places to use them is to link together two modifiers to make it easier for readers to understand what is meant. "Into bite-sized pieces" is clearer and easier to understand than "into bite sized pieces," so I think a hyphen is a great idea here. I can't say it's required, but this one of the things that hyphens do best.