You can see what fits in each of the four TTTHINGs by referring to the table below.
|NAME||DIMENSIONS||MAIN SECTION||EXTRA SECTION|
|Pocket TTTHING||18cm x 12cm||16cm x 10cm||16cm x 10cm|
|Pencil TTTHING||23cm x 10cm||20cm x 10cm||20cm x 10cm|
|A5 TTTHING||23cm x 16cm||20cm x 14cm||20cm x 8cm|
|Sketch TTTHING||29cm x 22cm||26cm x 20cm||26cm x 8cm|
As you can see I've included the outer dimensions along with the internal size of the main section/pocket and the exrtra section/pocket (with the printed lining) which is usually shallower.
If you'd like to check if something specific will fit you're welcome to get in touch to confirm.
It is also possible to order custom sized TTTHINGs if one of the sizes is slightly too small etc.