I keep one in my shirt pocket when I go to work. You never know when a hot chick at work wants to fuck after hours on the desk or a the floor. It is best to bring your own. Carry a variety latex and non-latex ones (some providers have latex allergies).  

Some providers will insist that you use their condoms. A good provider will have different sizes of condoms.  

One bp providers that I saw played a trick when I gave her the condom. She flipped the condom like it was card and could not find it. She used her own condom. We both got a good laugh out of it.

be sure they are in a sealed package

I typically have three sizes of condoms with me. Ultra thin, regular, magnum. I prefer to use the ones I bring because I know they aren't expired and I store and handle them properly. However, for an outcall to a person I have not seen before, it can get tricky. How many of each should I bring? I would say if you are a larger than average guy wanting an outcall, it might be in your best interest to have a few magnums on hand.

Never too small or too big and I control the condom.

Because I like Magnum bareskins the best and few ladies have those, other than my ATF who bought them just for me .

However, with my repeats, it's now always female condoms, because, while they look like shit, they feel like BBFS. I'd love to bring female condoms with me to new encounters, but that requires a bit of discussion, because not all ladies are down for that yet. Too bad.

but if you KNOW you require something other than just standard (or maybe have an allergy) then it's safest to bring your own and then explain. Not all providers will let you use your own - I've encountered a man that got off on the idea of getting women pregnant and was going to serious lengths to try. Just be upfront, she will either accept it or have something that works for you.  
xoxo