When you are loaded up you don't notice the weight or width when riding .. its only at very slow speeds that they become noticeable..
The size is up to you really, but people always take too much stuff, stuff that's not needed just wanted for a little luxury...
The width of the bike and panniers make no difference because when you filter you are very very aware that you are a "wide load" so filtering is taken with caution...
So it up to you what boxes you want...
As for pannier rails the givi rails wont fit, due to the little lugs on them..
So sw motec, touratec or basically any other make but NOT givi..
The fixings..Ohh dear, there are hundreds, most use the touratec pucks, search online for videos on the pucks..You can buy these direct from them as a kit..Or you can bolt them on directly to the frames if you wish..the only problem then is you will have to ride everywhere with them attached, then once you get home you unbolt them and put them in the corner to collect dust for another 12 months...