When taking doxycycline hyclate 100mg with food, how big or small should the meal be? And what should I avoid in the meal?

EXACTLY WHY???? How often? Details matter. Are you taking it for Lyme disease? Acne? For what? If you are taking it for acne the blood level is not important. If taking it for serious infection, then absorption can be a problem. Taking with food, especially calcium-rich food, may reduce absorption. On the other hand, doxycycline on an empty stomach may cause nausea.
Any meal and. lots of water or juice or even milk ok , just sit up for about an hour after because it has to be in stomach or if comes up can cause burning in the esophagus.