Most of these chains have nutritional info kicking about on their websites somewhere but Prezzo seems to have hidden theirs very well!
In those sort of cases, I'd probably do a combination of recall about how much I'd needed for another similar meal in the past along with a browse through Carbs & Cals to see if I can find a "lookey-likey" size-wise, and sort of make it up from there.
No idea what Prezzo portion sizes are like, but Carbs & Cals gives a starting point for six sizes, so let's suggest this one:
I'd also be factoring in things which have a bearing on the timing and whether a split is needed: carbonara, basically made of cream, so a fair few issues with absorption which would tend towards more attention to a split; bolognese, mainly tomatoes, little fat, so absorption is frequently more predictable.