Here is a simple guide for all levels and achievements. You can see the solution of each levels including the location of each tortoises and secret levels in the following video :
Timecode of levels and achievements
0:26 Tutoriel
2:20 Level 1 Seaside
5:22 Level 2 Desert
9:13 Level 3 Campsite
11:29 Level 4 Seasons
14:48 Level 5 City
17:38 Level 6 Themepark
20:27 Level 7 Space
24:00 Level 8 Halloween
26:50 Level 9 Golf
29:11 Level 10 Sakura
31:21 Level 11 Deepsea
33:58 Level 12 Medieval
36:30 Level 13 Farm
39:30 Level 14 Easter Isle
43:45 Level 15 Jungle
47:19 Level 16 Time Machine
51:48 Level 17 Mine
55:16 Level 18 Ants View
1:00:42Level 19 Swamp
Achievements :
1:05:29 A Sizeable Victory! : Complete the main levels of Sizeable.
1:05:40 Level 20 Northpole
Achievement :
1:11:00 Shellebration ! : Find every tortoise and unlock the secret levels.
1:11:15 Secret level 1
1:12:51 Main Menu
Achievement :
1:14:27 From the Beginning! : Find the secret level hidden in the main menu.
1:14:40 Secret level 2
1:17:28 Level 9 Golf
Achievement :
1:19:15 Hole in One! Unlock the secret golf levels
1:19:28 Secret level 3 Golf – Halloween
1:22:35 Secret level 4 Golf – City
