I take from these questions you are interested in selling stock photography as these days it is common to provide images digitally.
The answers really depend on what your client wants - mostly uncompressed tif files are the format that has been required but low compression jpgs are sometimes used. The format is dependant on whether they are concerned about whether the format is lossless or not (for jpg some information is lost every time the file is resaved)
again size is dependent on what they want the image for - many people sell images as varying sizes - web use can often only require a small file size whereas print requires more - look at some stock photography sites to see how they vary their prices according to size use etc - http://www.alamy.com is probably a good place to start.
getting it there - i have usually depends on the size required - if you and your client have broadband then and the file is not more than a couple of MBs then staright email - files larger than this sometime upload / download from or to a server or more likely CD.
