Math Time: Size Modifiers
Luckily for us, pathfinder has given us modifiers. If we believe that the people arrived at these modifiers using some sort of formula, we can use math to determine what size things should be.
For those who are interested, the 2-d Exponential (an exponential equation of the second degree) is:
$$mod = 0.727 - 0.101*x - 0.006*x^2$$
and the linear model is:
$$mod = -0.36*x + 3$$
You have to round to the nearest integer, of course.
This chart is me putting two fits on the data the d20 PFSRD has here. From this chart, you can justify a size modifier and a special size modifier of -27 and +27 (if you use the exponential model) or -19 and +19 (if you use the linear model), and a size modifier to fly of roughly the same number.
You can do roughly the same thing with the other values, but I don't encourage it. See the "easier solution" below.
The problem here is that the d20 pfsrd does not mention anything more than colossal, so whatever you choose to do with these larger sizes is a guess!
Stealth Modifier By Category
It appears the size modifier to stealth is based off of how many size categories they are from medium, multiplied by 4. So if you immediately tack on this super-colossal on the end, you get -20.
These basic rules can be used if you bother to account for things by size category, but then you need to figure out if you need a size category between colossal and this other size.
Another, Easier Solution
Sizes in combat are a rough approximations of how much room a creature takes up while fighting. Out of combat, creatures squeeze or spread out as needed, so the 60' wingspan may just be when it's flying.
Taking a look at Quetzalcoatlus from a few sources:
We see it has a 4-legged stance when it's not flying, which certainly reduces the horizontal area it takes up. This 4 legged stance may be it did when it wants to fight or eat something. It would be a safe assumption that dragons and other creatures would do something similar.
From the pictures, it looks like it's giraffe-sized when in a quadrupedal stance, so looking at the modifiers of a giraffe may be a good starting point. (That makes it a huge creature, and happens to agree with the given one.) The long beak could merely give it a "reach weapon" fitted for its size.
Your proposed fly speed would make anything but the largest of gaming tables ineffective at capturing the size and speed of such a creature. It may be wise to forgo showing the size of this creature on a grid until it lands.