I'd be communicating mostly for / to PayPal at this stage, while keeping to the dispute messaging system. Keep it short, bullet point style of communication by pointing out exactly what you want the PayPal reps (who may have to decide on the outcome) to know.

Stuff like, for example, "I have carefully examined the photo provided by [buyer], and would like to note that it confirms the size of the item received matches the size of the item purchased, due to the following reasons...", and then I would cross reference what is shown in the photo, to the relevant part of the description, and so on. 

Unfortunately PayPal are currently only offering an automated messaging system in place of their call centre at the moment, so it's unlikely you'd be able to talk to or with a CS rep directly to get their advice or guidance - I'm pretty sure it just scans your enquiry and spits back copy / pastes of whatever it thinks is the right policy or help page. 

(PS, this is all assuming you are not on board with having the item returned for a refund - you will most likely have an option to accept a return of the item to refund the buyer, but it would be 100% of their original payment, including postage, and you won't be refunded any of the fees paid to PayPal. If you would rather not agree to that, personally I would not escalate unless I thought it necessary and / or I was confident I had laid out my statements and evidence well, and that it would be almost guaranteed to be decided in my favour, which is quite difficult to be certain of. There may be a reason the buyer has not escalated (not confident of success), or they may not know they need to, but escalation is more for the buyer than it is for the seller without knowing how it will go (that is, for a buyer dealing with a seller who won't help, they get PayPal to decide one way or another, it's the only way they'll have a chance of getting what they're asking for. On the other hand, for a seller, nothing will happen, or be forced to happen if the case doesn't escalate, so the only thing escalating will do is have it over and done with one way or another more quickly, since there is a period of 20 days after a case is opened where it can be escalated. After that 20 days, though, it will just time out if not escalated).