The Cisco SG300-10 says its forwarding rate is 14.88Mpps with 64 byte packets. I estimated this to be ~ 7 Gb/s.
Does this mean that up to 7 computers can transfer data across VLANs at full 1 Gb/s wirespeed (assuming 64 byte) packets?
Why would you ever send line-rate 64 byte packets in any scenario other than a switch drag-race? That said, cisco's data sheet numbers indicate it can do unidirectional 64-byte line rate for at least seven computers.
I'm assuming you understand that ethernet's IFG and SFD mean that you get nowhere near 1Gbps of real data @ 64 bytes.
If you're using FTP, HTTP, CIFS/SMB, H.264 or any other protocol used for data transfer, then yes 7 computers can do bi-directional line-rate based on Cisco's quoted 14.88Mpps and 20Gbps of "bandwidth" in the data sheet
How will this vary with packet size? What kind of actual throughput can I expect when multiple computers are transferring files using SMB across VLANs?
Under normal circumstances the SMB protocol is not limited by the network. It's limited by how fast you can unspool data from your server's disk.
does it do wirespeed intervlan routing?
Yes up to 14.88 Mpps