Being 5"9 & about 330 lbs I fly across Europe and neighboring countries about 3 times each year.
Flight duration is between 3 and 5,5 hours.
Always economy class. (distance between seats about 77 cm - 2"6 when I converted correctly - anyway - not much legroom)
I am usually asking directly when entering the aircraft for a seat belt extension.
I pre-book seats at window or aisle, I ask cabin crew for re-seating when flight isn't fully booked and empty seats are available.
[edit: sometimes the person sitting next to me asks for that before

]
Pre-booking of an additional seat is an alternative too. Most companies offer this.
I would only recommend to do so when fares are low - otherwise it is way too expensive- that means booking early or/and choose flights with low demand. (for example not the best departure times, not high season flights - all depends on local or regional circumstances)
Is it comfortable to fly in my Dimensions?
No, it isn't.
But I want to go and see the world and its different places and cultures - therefore I neccesarily need to use this mean of transport, even if it is quite uncomfortable to get to my destination.