Head to ''Campaigns'' in the main navigation bar at the top of your account and then click on ''Forms'' in the drop-down.

Select the form you'd like to edit (click the little pencil icon on the right-hand side)

When you're in the form builder, scroll down to ''What fields would you like to display in the form?''. You will see a tick box called ''drop-down''.

By selecting this, you will then see the option to capture multiple choices. Here you can add in various T-shirt sizes.