It's related to your size. Most manufacturers will have a chart tying up the two. For touring / recreational paddling I would even consider going down a size to keep it chilled and easy.
Hi Spirit... it's been a long time, I hope you are well!
I like a 6.5" wide blade with straighter sides because it is easy on the back and shoulders, plus it really improves my cadence for quick acceleration when catching waves.
Mine is 85 sq inches
It is the same paddle I use on flatwater for the same reason.
I've got black project surge medium for surfing. I used to use fanatic 7.25" for surf and flat water. But recently i got a starboard lima in large for flat water. Initially I found it hard on my shoulder with the large blade. Within a month I'm used to it & liking it. & when I'm out surfing with the surge i find it even easier & more lightweight than ever & not getting fatigued