What's a good size/resolution when showing pictures to client using a online tool for proofing ?
I've noticed that 1350x900 and 72ppi is pretty good, considering image quality and size on disk.
What more do you suggest?
Short answer: it depends on lots of things...
To me, proofing suggests not zooming in to 100%, or even zooming in at all larger than screen size, so a typical screen size (which may depend on the client/proofer) seems like a good starting point to specify an absolute upper limit.
Since it's online, my advice would be to have two viewing modes:
Have a look at how, say, Flickr sets up their users' photostreams; rows of photos around 300px high, which can be clicked on to view as a single large image. Facebook does something similar with albums (though both thumbs and 'full size' are quite a bit smaller on there). I'll bet 500px, imgur, and the countless other image/photo hosting services have similar album vs single-shot views and default sizes, so see how they do it, and what sizes they've chosen, and what upper/lower limits you think fits your application.
If you care about potential lost revenue by clients using the previews directly, you'll want to watermark the Previews quite noticeably, and/or limit images to say 600 px max dimension so they won't be usable as high-resolution prints or wallpapers. Even at 600px you can easily see the content of an image (all photos on Facebook had a max dimension of ~650 up to a year or so ago), if that's all that's required.