The size depends on the location
where you want to place the painting 

 It is not always easy to imagine 
To help you, here are some examples  
of size with objects that have a standard size 
To lay the painting on a desk 
size 30 cm x 20 cm (12" x 8")  seems a good choice 

To hang the painting in a large room wall 
it is better to choose a larger size 
120 cm x 80 cm in this example 
The little painting on the piano  
is 40 cm x 30 cm (16" x 12") 
you see this is not the best size
to hang on this wall

You can tape several sheets of A4 paper 
they are approx. 30 cm x 20 cm (12" x 8")
to have a view of the real size