How to transform oxalic acid small crystals into large crystals?
Like with most water-soluble compounds, the easiest way is to dissolve your oxalic acid in hot water and then cool down slowly. As the solubility decreases with cooling, much of your compound would precipitate, forming nice crystals (or not so nice, depending on the speed of cooling and the cleanliness of your glassware.)
Dissolving and letting the solvent to evaporate also works OK, just too slow (in case of water). As for melting, this is inherently bad idea for just about any organic compound. Even if you can keep your temperature precisely so as to avoid decomposition, you'll end up with an ugly lump of molten and recrystallized substance, probably containing some large crystals inside it, where they can't be seen, much like a beautiful sculpture hidden in a chunk of marble.