Titan art sizes are nonsense, if i remember correctly when Adeptus Titanicus came out Warlords were supposed to be 80 feet tall.
If anyone wants to build bigger Titans for Epic then that's fine but if they want them to be more powerful than an Imperator in game then i would suggest they have them as a unique creation like "The Spike" or the Ordinatis weapons and work them into a narrative otherwise you just end up in a size war where someone brings a 40k titan to an Epic game and the other titans end up with no value.
When titans were first introduced my first thought was to convert a star wars "chicken walker" to a Chaos titan for Epic but i'm glad i never bothered as it would look stupid.
I think Ork Gargants have the greatest potential for going overboard size wise without stepping outside the background, i have often pondered making a half built uber-mega Gargant to act as the objective in games-you could choose to have it totally inactive or treat it as a fortification with some working defences.