This is a difficult one. I think the solution lies not with Inkscape but with your GTK theme. The best lead I can give is to point you to the gtkrc files which may be found somewhere on your system. (I'd give more specifics but I am not a Mac user.)
GTK? Sorry, you've lost me there.
The size of objects seem to change as one changes the thickness of their outline - seems odd to me - it makes aligning things a bit 'hit and miss' if you then go back and edit them later. Admittedly, Xara had a similar (but not identical) issue - it's the one and only area where I think Illustrator is superior.
That is a configurable behavior. There's a button for it on the toolbar when the Selection tool is being used. This might help:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Transforms.html
That's not actually what I was asking - but thanks to the clues therein, I think I am now on the way to the answer...
The W and H boxes show the size of the bounding box and not necessarily the size of the actual object (if you know what I mean) and when you snap an object to another, you always snap bounding boxes, rather than the objects themselves - so if you start with a 250px square, add a 5px outline, you get a 255px bounding box. Bit of a pain if you're doing precise technical illustrations, because things don't tile properly and you end up with outlines alongside each other, rather than overlaying.
But...
After some more fiddling...
It all depends on how you set things up in the Document Setup dialog - setting the options to 'Snap nodes to objects' rather than 'Snap bounding boxes to objects' (the default?) it works as I want it to - i.e. that outlines don't affect the way things snap. COOL! I like it. That is *better* than Xara - you can't do that in Xara - snapping is always governed by the bounding box (i.e. object + its outline).
Thanks, Phil