It all depends what the final use is!
Generally I tend to ask. No point in supplying a 35Mb image for a 1/4 page in a magazine. However don't be surprised if the answer is "full res" or "full size". as often the client may not know. In which case that's what you send. Also ask do they want Tiff or jpeg. It's usually jpeg.
As far as resolution is concerned, in most cases set that to 300dpi, unless requested otherwise. It makes no difference to the file size but it may help when it comes to the final design of the page.