Check the “make your own game” category here on the page. The dimensions are there (can’t link them atm, sorry.)
EDIT: Hah, got it:
Icon: 1024×1024, but mobile devices will resize it down to 48×48. This icon should NOT include the name of your game.
Splash Screens: This screen will be displayed briefly as the game starts (on various devices) and will act as the first screenshot in the store.
480×800 portrait (Android)
640×960 portrait (iPhone 4)
1242×2208 portrait (iPhone 5+)
1536×2048 portrait (iPad portrait)
2048×1536 landscape (iPad landscape)
Other Promotional Art: These landscape images can include the name of your game in a large font.
1024×500 landscape
1400×560 landscape
480×320 landscape