honestly from what i have found a lot of them are fairly standard. if you end up with ones too big cut them, and build a new edge with the sealer bit in the vac machine. I would say to call the company and ask for a sample so you can test out the quality. especially if it is someone you work with quite a lot you can get them to bring some by.
Do you use a chamber vacuumer or the smaller tabletop sort which require embossed bags?
For my kitchen the meat comes in as whole carcasses, with the offal removed ( for red meat I currently have lamb, reindeer, bear, elk and wild boar on the menu) . After cutting and separating the cuts, I sort and vacuum them whole. As I only take in one group of customers at a time, I don´t really need to bag the meat in portion sizes which would be a good idea if you´re running an a la carte restaurant.
Basically I wouldn´t use any bigger bags than is necessary. If you´re using a chamber-vacuum, keeping the pressure down while vacuuming fish helps, as high power bruises the texture, especially with the more delicate sea-food. As for the thickness of the bag, it doesn´t really matter unless you´re vacuuming something on the bone, as those might pierce the bags. I usually wrap a bit of cling-film or folio at the end of ribs to prevent that happening.
Not all bags tolerate high heat, and some manufacturers only guarantee their product to tolerate at most 70 degrees celsius. For most proteins cooked sous-vide, however, that temperature is more than enough. If you´re new to cooking sous vide I suggest finding some tables for the temperatures and times required for different ingredients.
Under pressure is a great book by Thomas Keller that´s solely about sous-vide cooking, and if you´re only going to buy only one cook book to last the rest of your life, I must recommend the Modernist Cuisine. It has lots of avant-garde methods described in it, which some might condemn molecular trickstery, but even if you shun from that sort of stuff, the depth of information presented in the book(s) is astonishing. Expensive, sure, but worth every penny.
Doing a quick googling I found this UK based website. The prices seem fair enough for me, but then again, I´m living in Norway... And nothing´s exactly cheap around here.