Three rules decide whether your list works
A spelling list and a good word search list are not the same thing, and the generator applies three rules that are not obvious from the screen. The warnings above the sheet tell you when one of them bites, but it helps to know them before you start typing.
Words under three letters are dropped. A two letter sequence turns up by accident all over a grid of random letters, so a child asked to find AT finds it in a dozen places and none of them count. There is no setting for this, because there is no version of it that works.
A word cannot be longer than the grid is wide. Fourteen letters on the 14x14, eight on the 8x8. Spaces do not count: NEW YORK is seven letters as far as the grid is concerned, and it appears spelled properly on the word list even though the space disappears between the cells.
Each size holds a fixed number of words. Twelve on the 8x8, sixteen on the 10x10, twenty on the 12x12, twenty-five on the 14x14. Give it more than that and it draws a different subset every time you generate, which is the feature behind a class set: thirty children, thirty different grids, one vocabulary list.
What makes a list produce a good grid
Mixed word lengths beat uniform ones, because the long words act as landmarks that make the short ones easier to find afterward. Varied first letters matter more than people expect, since the first thing anyone does is sweep for an initial letter. And words that share letters in the middle knit the grid together, which is why a list built on a pattern always feels denser than a list of unrelated nouns.
There is more on all three, with the numbers, inhow to write a word list that makes a good puzzle.
If you would rather not type a list
The topic buttons above mix a ready-made bank into your own words, and you can remove individual entries you do not want. If you only need a sheet on a subject, theprintables are already written, with the classroom sets grouped separately under worksheets.