First of all - if you require an alternative size please ask, I can resize most designs and can print anything up to A3 size. More detailed information, e.g. on choosing a frame,
can be found at the link below

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Picture Frame Size Guide

As for the standard dimensions given in each offer:

Vertical layouts:

 A3 --------> 29,7cm x 42cm | 11.7″x16.5″ 

12x16 -----> 29,7cm x 40cm | 11.7″x15.7″

11x14 -----> 28cm x 35cm  | 11″x14″

10x12 -----> 25cm x 30cm  | 10″x12″ 

A4 --------> 21cm x 29,7cm  |  8.3″x11.7″ 

8x10 ------> 20cm x 25cm  |   8″x10″ 

6x8 -------> 15cm x 20cm |   6″x8″

5x7 -------> 13cm x 18cm  |  5″x7″

Horizontal layouts:

A3 -------- >  42 cm x 29,7 cm  |  11.7″ x 16.5″ 

16x12 ------>  40 cm x 29,7 cm  |  15.7″x11.7″ 

14x11 ------>  35 cm x 28 cm  |  14″ x 11″ 

12x10 ----->  30 cm x 25 cm  |  12″ x 10″ 

A4 ---------->  29,7 cm x 21 cm  |  11.7″ x 8.3″ 

10x8 ------->  25 cm x 20 cm  |  10″ x 8″ 

8x6 --------->  20 cm x 15 cm  |  6″x8″

7x5 -------- > 18 cm x 13 cm  |  7″x 5″

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