I've had Borders for 13 years now and not seen one that would be my Ideal.
Dog out the back is a nice size and game enough for a young pup ( 8 month ) but nothing compared to my first one who would hunt anything, anywhere and not give an Inch but was too deep in the chest and a bit slow ,below, to ever make anything great.
I hope the lad out back proves me wrong, in fact I know I am being hard on him.
Back to what the Welsh bloke has ask'd, I reckon about 13 - 14", easily spanned with a hard jacket and looking balanced between head size and body.
Of a dog, I would expect, like