First of all, your modem and ethernet card are not going to speed you up any. That's based on your ISP and may also be related to phone line or cable line quality (depending which you use). It is not like dialup, where there are different speed modems.
Second, if you're having problems from one sender, it's most likely that sender who is giving you the problems. Try others.
Third, you cannot set the packet size to 99999. Even if it let you, it would cause problems rather than doing anything to speed things up. You have to understand how packetsize affects transfers... Packet size is used to handle data being transfered. It wouldn't make sense for it to check every byte as it is being sent, so you check each packet instead. If you check 4kB packets, and you "drop" (lose) a packet, you have to redownload the packet. That means redownloading 4kB. If it was a 2kB packet, you'd only be redownloading a 2kB packet. The difference is minor for one packet, but can increase quickly if you drop packets often (or the sender does). So higher isn't always better. On the other hand, lower isn't always better either as you have to check more frequently which can slow things down. In the end, most people get the best speed at 4096. Certain people get better speed at 8192 or 2048. You won't know without trying. Of course, trying requires a LOT of tests from a LOT of senders to really know. Just testing a couple of times from a couple of senders won't really give you valid data. In any case, the difference in speed is minimal.