Think you’ll be more than OK with 2 up front and seats down and maybe OK with seats up if soft bags (well maybe a bit of leaning forwards).
BTW, if you do have a choice, the current Octavia has a larger boot than the current Mondeo according to AutoCar:
Boot space
With a keen eye on practicality, every Octavia is better at load carrying than the equivalent Volkswagen Golf, but the estate is a luggage-lugging tour de force, able to hold 610 litres of freight. That’s a figure which can only be beaten by the 680-litre Peugeot 308 SW. However, we still rate the Octavia as the better all-rounder.
This huge amount of space can be further expanded by dropping the rear seat back, reaching 1,740 litres and beating certain cars from the class above – the Ford Mondeo and Mazda6 for example. There is also a useful false floor
Mondeo Est 525L / 1,630L
Octavia Est 610L / 1,740L
Passat bests them both if that’s an option? Or as trail_rat says, try to find a Berlingo?