Some answers can be found in the FAQ list linked to at the front page.
The data packets are currently 12MB in size, but they are used for multiple Results, and the scheduler should send you Results that match the files you already have. However, the number of Results remaining for a certain data file gets very low at the end of an analysis run, so the number of files that neede to be downloaded increases there.
The result file is zipped before being uploaded, so it's usually quite small.
The size of the WU is roughly set so that a modern machine which contributes 50% of its CPU time to Einstein@Home should be able to finish a Result per day. It depends a lot on your machine and, sadly enough, on the operating system.
BM

WU upload
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Download file sizes are currently 6.3 MB (l1_xxxx.x) and 8.3 MB (w1_xxxx.x). Average upload size is 140kB but it varies from 220 Bytes to 15 MB in a few cases.