I'm afraid that on the network point of view this is a somewhat impossible question.
In wired Ethernet if you have a Gigabit switch and 2 clients with GB NICs, you can expect to get a stable ~1GB/s throughput rate. In the wireless world the advertised max throughput rates can only be achieved in a strictly controlled lab environment, in any real live environment many factors affect your throughput:
- Wireless is a half duplex medium
- Wireless is a shared medium. The more clients you have on the same radio, the less bandwidth is available for each client.
- The point above is theoretical, distance affects. If your clients are both 2m from the AP, you get throughput n. When one of the clients moves to 4m away from the AP, your throughput drops.
- There's background noise, and there's interference from both WiFi and non-WiFi devices. This means that if you get average throughput of n Mb/s, I won't get the same throughput even using exactly same equipment and drivers you use due to only different noise floors. Even you may see different throughput in the afternoon compared to the morning. If you use a wired mouse and mine is btooth, we're likely to get different results. Lighting fixtures can interfere wireless throughput. And so on.
- You're using n, where "max throughput" becomes a complicated matter. It depends on what modulation scheme is used, how many spatial streams you have, guard interval etc., answer being somewhere between 6.5 Mb/s and 600Mb/s... same applies to ac-networks.
- If you have legacy clients on your wireless network, the average throughput will drop
I'd suggest you put this question to Stack Overflow. The coders might be able to give you a more meaningful answer as someone is likely to have tackled similar situation in the past.
Of course you can just take the expected average throughput rates for example from http://www.speedguide.net/faq/what-is-the-actual-real-life-speed-of-wireless-374, but those tend to depend on whom you're asking. Here's a discussion from Superuser concerning wless average throughput on 802.11n: https://superuser.com/questions/187485/average-transit-speed-for-802-11n-wireless-ap. Still you should ask about calculating appropriate buffer size in Stack Overflow.