Can you please tell me what size stroller insert it takes - small, medium or large, or is there a specific size?
Do you mean an insert as a liner, this pad for the child to sit on? If so, I would suggest a medium size (not overly wide), as the seat is, apart from not being extremely wide, quite normal. It's more the chassis that is small.
I would suggest an Elodie Details seat liner, an Aerovmoov pad, a Diono liner, or anything universal, really. The brands offered quite vary from country to country.
If you need a seat insert for a small baby, do check the Aeromoov Cosymorpho or similar, or the Easywalker Soft Cot.