You won't have a problem with the majority of providers as others have said above, but i definitely understand the concern. It's one i had when i first started this too. If you have an attribute that's not what they are most used to working with it'll help to let them know ahead of time so they can make accommodations if needed. The providers also will not find it weird if you don't want to get fully naked, you don't have to. But that again is something you may want to tell them ahead of time. Especially if the provider you're hiring is going to be performing a massage. That's generally not something done with a shirt on.
At the same time, CMNM (clothed male, nude male) is a kink/fetish people have where you engage in the usual "activities" but one person stays clothed. This is something many providers (particularly ones who mention being kink friendly on their profiles) will likely already be familiar with so it shouldn't be an issue. As others have said, providers are used to a wide range of body types and age ranges. The best thing you can do is just be hygienic for them. A thorough brushing of the teeth + mouth wash and thorough bath/shower before the meet goes a LONG way. A provider will prefer a client who's obese and clean over a fit guy who hasn't showered in 3 days every single time.