Ok, no Latin or lurching!
The seed/pip/stone/nut thing ... It's more than size.
"Seed" covers all pips, grain, nuts etc and means the ready-to-grow embryo of a plant.
"Stone" isn't about size; it's about hardness. Dates, apricots, avocados, cherries, lychees and many other fruit have really hard, often woody seed-coverings. You couldn't bite into one without hurting your teeth.
"Pips" are more fleshy. You can bite into them quite easily (and don't they just taste yucky, in many cases!)... grapes, apples, pears and so on.
"Nuts" are seeds with an edible kernel.
And as for fruit and veg... technically a fruit carries seed. So strawberries, cucumbers, tomatoes and pumpkins are all fruit.
It doesn't matter whether the seed's on the inside of the outside (I wonder if Merciasound's thinking about the technical definition of a berry, because horticulturally that only refers to fruit with seed inside, so not strawberries.)
But that's horticulture -- it's a science. :D
As far as cookery's concerned, of course, it's down to tradition... and tradition's not always logical, so you can't even say that fruits are sweet and vegetables are savoury (tomatoes are sweet, and so are carrots!) ;D
My dad used to say "Cleverness is knowing that a tomato's a fruit. Wisdom is knowing not to put it into a fruit salad."