It will be ok for a couple of years but I would suggest the biggest pot available and use John inns no 3 (soil based compost) its a thirsty plant as well so keep well watered especially in the summer

How tall is it now ?

Gg

Its a minature one it will only grow to about 4 ft

A Kilmarnock willow will grow a lot taller than 4 foot! Can get up to 8 foot in height.

you can of course bonzie it but youd have to trim the roots etc .

I have 2 planted in the front garden I have had them for six years and on is 5ft and the other is 4ft and they bush out but never gain height

As they are grafted, they will only get slightly taller than there original height but will put on width and root growth. As they are salix (willow) they will always want plenty of water so best grown in the ground.

Hi, I think the height of a Salix caprea 'kilmarnock' is between 5 and 6 ft, depending on the grafting height, like most plants it would be better planted straight into the ground, so that it could spread it's roots out in search of water and nutrients, but if you have to keep it in a pot, the larger the better, and keep it well fed and watered, Derek.

most plants are better in the ground .