Patrick McLoughlin, the transport secretary, is preparing to pay millions of pounds to turn first-class vehicles to standard-class on First Great Western trains operating into and out of London’s Paddington station.
The simple answer to this is to have a reasonable standard seating density in a single type of vehicle and change the class designation as required. A mark 3 coach will accommodate 76 passengers and their luggage comfortably, which was the arrangement before their most recent refurbishment. The virtual image above illustrates an interior with a mixture of facing bays and airline seating. This gives the same number of seats in a slightly shorter vehicle than the mark three, which could therefore be slightly wider, built almost to the full width of the loading gauge.
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Surely you are old enough to remember that "standard class" is in fact "second class", or perhaps even old enough to recall that "second class" was really "third class" - I am sufficiently nostalgic to say "2nd class" as a point of stubbornness.
In other news - you should probably read http://www.telegraph.co.uk/news/uknews/road-and-rail-transport/10551060/HS2-Government-ministers-sat-on-critical-report-by-Department-for-Transport.html if you haven't already..
If the vehicles are built to the full width of the loading gauge, they can be 2.82 metres wide externally, ie 2.65 internally from floor level up to elbow height of seated passengers. I would suggest that this is enough for anyone, but it does mean that vehicles probably cannot be more than about 22 metres long. If you want to know what this feels like, it is more or less what you get in a class 180.
Unfortunately you can not have it all ways, something has to change to cope with the numbers of passengers in the UK. Tinkering with coaches (which seams to be your answer) will not get very far.
Also bear in mind the width per seat when the total internal width of the train is 2.65 or 2.57 (2.74m external) is 2cm, but that is still wider than standard class seat and about mid range for first class seats on any aircraft.
There is an arithmetical error in your last paragraph, you might like to correct it and re-post. The standard external width on eg the 26M long French Corail stock is 2.85. Swedish stock is typically 3M to 3.5M wide with 2+2 seating on the inter-city versions. 2.80M width seems a reasonable target with the thinnest practicable bodysides. That is the width of stock such as the class 375 Electrostars, and although these are a nominal 20M long, a slightly longer vehicle appears to be possible at this width without the need for tapering.