One caveat here. The weight and the length of a spey line are interdependent on each other. Locking down on a perfect weight first, only to alter it's length later, may require a minor change in weight class. But at least you have 4 digits to the zip code to get the 5th easily.
Let's start with the weight because this is critical in properly loading your rod of choice. Given a fix rod, the suitable choice of line weight- given the same taper and density design- is now left to be determined by the casting tempo and power you are most comfortable performing for long lengths of time. Too much power to properly load will tire you out first, too fast tempo will wear you out next. So, the trick is to repeatedly cast from a window that will neither tire nor wear you out first, but rather feeling tired and worned out at the same time (the equal drawn down of both means you have gone as far as your casting stamina has allowed you). So, the perfect weight is like finding a favorite book you enjoy going back to repeatedly. It neither reads like quantum physics (requires too much to power through) nor is it too watered down and predictable (requires too little effort making you read fast). The books ends with close to equal drawn down of both power and reading appetite.
What is too fast of casting tempo causing early wear out? This is the tempo that disproportionately wears you out first before it tires you. Simply underline your single handed 5 weight rod with progressively increasing weight class drop. By the time your reach 2 or more class drop for proper loading, you will find you are casting like a marooned ship wreck survivor in frantic need to be noticed. Soon, the survivor has to stop not because his arms are heavy from applying power, but from the wear and tear of waving back and forth too fast.
Once you found your weight, the next is to achieve the proper length (to be continued in part 2).