Ever try to keep it within proportion with each other? Just a moment while I crunch a number down.
Okay, here's what I figured so far:
Resolution divided by tileset = functional grid scale. It must be in proportion, and to adjust them a tad may require a bit of tinkering.
Summary: 1440x600 / 12x12 = 120 x 50
(1440 / 12 = 120) (600 / 12 = 50)
According to tileset, each tile is natively 12h12v in scale So, just merely changing grid alone without following up by adding or subtracting 12 pixels to the resolution will result in the silly putty effect and it'll get all stretched about.
I think I see the problem. If you want to reduce the window size, just proportionally increase/reduce the scaling.
IE- For every 1 tile of space you want added/removed, add or subtract 12 from the resolution vertical or horizontal for each tile.
Now, in regards to making everything look nice and square, the larger the resolution, the easier it should be to mess around with. I advise you screw around with the grid settings first before screwing around with the resolution; then again, the resolution settings alone should be much less fickle.
Conclusion: When you went 119x50, it stretched from it's original horizontal scale by 12 pixels, resulting in a stretched/flattened image per tile.
I find it up to you to crunch the numbers down to square-out the tiles. Actually, if you want to keep the amount of tiles the same, but make it more square, adjust resolution exclusively, not the grid.
I recall using this kind of measurement and adjustment to make DF easier to play since I can't adjust the resolution of the game from the game.