Nearly any size is possible. 
I work in standard 'A' paper sizes for my illustration and Watercolour work. Please see the image and sizing chart for full sizes on those. 

 

For the Simple Illustrations Watercolour; 
A7 - Single Pet Portrait is £14.99

A6 - Single Pet Portrait is £19.99

A5 - Single Pet Portrait is £24.99

A4 - Single Pet Portrait is ‎£34.99

A3 - Single Pet Portrait is ‎£44.99

A2 - Single Pet Portrait is ‎£54.99

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£15 extra per pet if wanting multiple pets, a rider or adding a person into the picture.

 

 

For the Watercolour Portrait Pieces- Currently 10% off;

A7 - Single Pet Portrait is £23.99

A6 - Single Pet Portrait is £28.49

A5 - Single Pet Portrait is £33.24

A4 - Single Pet Portrait is ‎£42.74

A3 - Single Pet Portrait is ‎£52.24

A2 - Single Pet Portrait is ‎£85.49

A1 - Single Pet Portrait is ‎£124.44

 

£15 extra per pet if wanting multiple pets, a rider or adding a person into the picture.

Jumps, carriages or any wording is £10 extra

For the Oil Paintings I offer them in a set size/price list of; 

 12” x 10” - Single Pet Portrait is £285.99

14” x 10” - Single Pet Portrait is £400.00

16” x 12” - Single Pet Portrait is £625.00

18” x 14” - Single Pet Portrait is £885.00

20” x 16” - Single Pet Portrait is £1000.00

24” x 18” – Single Pet Portrait is £1325.00

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£175 extra per pet if wanting multiple pets, a rider or adding a person into the picture.

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However if you are wanting some thing at a custom size do just let me know, as most sizes are available.