If as you say the boxes on the ESA50 that you have been sent are too small, you could simply write in the relevant boxes "see attached sheet" and write or type your answers on A4 size paper. If you decide to use this method, ensure that your name and NI number are recorded on each sheet before stapling them to the ESA50.
Alternatively as you have alluded to in your post, you can use the interactive PDF version of form ESA50 if you prefer?
Nothing on this board constitutes legal advice - always consult a professional about specific problems