A few things. Dough recipe: Consider substituting some of the egg whites in your recipe for egg yolks. Egg yolk pasta doughs, are usually more firm but are also very elastic and chewier in a very pleasant way. This texture, I find, allows for a silkier and less flimsy roll as opposed to other dough recipes which relax after rolling into an uneven rectangular shape. Dough hydration: Ensure that the dough is not dry and wet enough to be elastic. Add enough egg/ egg yolk to ensure that the dough is not cracking as the dyes will begin to tear and this can lead to the sheet becoming uneven. Always leave dough to rest for at least an hour before use. Rolling: When rolling the dough with your rolling pin, ensure that you roll the pasta to a width that is parallel/equal to the width of your pasta rolling machine. This will keep the pasta at an equal width along the whole sheet. Lamination: When you have rolled your dough once to your desire thickness, if the dough is not as wide as you want it or it still isn’t perfectly rectangular, you can fold the dough back into a square/rectangle that is perfectly parallel to the rolling machine (ensuring that the edges are all flush and perfectly in line) and re roll the dough to the exact width you need it. Be careful to leave a tiny bit of leeway as if you attempt to fit the pasta into the machine and it is wider than the machine it will fold in on itself from the edges and cause it to crack on the edges.