To fold a paper airplane, Austin starts with a square paper $F OLD$ with side length $2$. First, he  folds corners $L$ and $D$ to the square’s center. Then, he folds corner $F$ to corner $O$. What is the longest distance between two corners of the resulting figure?
1. **Initial Setup**: We start with a square paper \(FOLD\) with side length \(2\). The center of the square is the midpoint of both diagonals, which we will call \(C\).

2. **First Fold**: Corners \(L\) and \(D\) are folded to the center \(C\). This results in two new points, \(L'\) and \(D'\), which coincide with \(C\). The paper now has a crease along the line segment \(LC\) and \(DC\).

3. **Second Fold**: Corner \(F\) is folded to corner \(O\). This fold creates a new point \(F'\) which coincides with \(O\). The paper now has a crease along the line segment \(FO\).

4. **Resulting Figure**: After these folds, the resulting figure is a trapezoid. The bases of the trapezoid are the segments \(L'C\) and \(D'C\), both of which are \(1\) unit long (since \(L\) and \(D\) were folded to the center \(C\), which is halfway along the sides of the square). The other base is the segment \(FO\), which is \(2\) units long.

5. **Longest Distance Calculation**: To find the longest distance between two corners of the resulting trapezoid, we need to consider the diagonal of the trapezoid. The longest diagonal in a trapezoid is typically the one that spans from one vertex of the longer base to the opposite vertex of the shorter base.

6. **Using the Pythagorean Theorem**: The longest distance will be the diagonal from \(F\) to \(C\). Since \(F\) is at \((0, 2)\) and \(C\) is at \((1, 1)\), we can use the Pythagorean theorem to find this distance:
   \[
   \text{Distance} = \sqrt{(1-0)^2 + (1-2)^2} = \sqrt{1^2 + (-1)^2} = \sqrt{1 + 1} = \sqrt{2}
   \]

However, this calculation seems incorrect based on the problem's description. Let's re-evaluate the longest distance considering the correct points.

7. **Correct Longest Distance**: The correct longest distance should be between the two farthest points after the folds. Given the trapezoid's bases and legs, the longest distance is actually the diagonal spanning from one corner of the longer base to the opposite corner of the shorter base. This distance is:
   \[
   \text{Longest Distance} = \sqrt{1^2 + 2^2} = \sqrt{1 + 4} = \sqrt{5}
   \]

The final answer is \(\boxed{\sqrt{5}}\).