I guess it depends on how you measure the two screens.

KDX screen is 1.61 times longer in the diagonal

The K2 has a 6" diagonal length as compared the the KDX 9.7" diagonal length, so that is not double the size in the diagonal length.

The KDX has a screen diagonal 1.94 times longer than a 5" screen.

KDX screen is 2.47 times larger in surface area

The K2 screen is 3.6" x 4.8" for an area of 17.28 sq.in., compared to the KDX screen at 5.4" x 7.9" for an area of 42.66 sq.in. That means the KDX has a screen 2.47 times larger as for as area goes.

A 5" screen is 3" x 4" for an area of 12 sq.in., and that means the KDX has a screen 3.56 times larger as for as area goes.

KDX screen has 2.06 times more pixels

The K2 has 600 x 800 pixel resolution for 480000 pixels compared to the KDX screen resoluton of 824 x 1200 pixels for 988800 pixels. That means the KDX screen has 2.06 times as many pixels as the K2 screen.

A 5" screen still has 600 x 800 pixel resolution, so the comparison of pixels is the same as for a 6" screen

Conclusion

Since area and resolution are the important factors for me, I would have to say the KDX screen is more than twice the size of the K2 screen, or any comparable 6" e-Ink screen.