According to Aeroplanes of the Royal Aircraft Factory
by Paul Hare, the dimensions were as follows:
Upper Span 57' 2.5" (see comment below)
Lower Span 45' 3.5"
It would appear that the 5th and 6th production machines had upper wing extensions. This was the variant (No.380, 6th machine) that achieved an altitude of 18900' on 14th May 1914. As Mossie says, the front cockpit seems to have been sheeted over. "Normal" RE5s could reach 17000'.
Speed is given as 78mph, initial climb 400ft/min
Power unit was a 120hp Austro Daimler
Bit more from Jack Bruce's "Aeroplanes of the Royal Flying Corps (Military Wing)":
the 11th and 13th machines had extra fuel tanks and the 13th machine had the upper wing extensions. the 16th was fitted with a thing called the Forbath stabilizer, and the 15th was fitted with air brakes. The 18th was fitted with the Royal Aircraft Factory's oleo undercarriage and the upper wing extensions. The 20th and 21st machines were fitted with wireless, and the former machine had an enlarged fin. The 22nd machine had the oleo undercarriage and a jettisonable fuel tank and was re-engined with a 140hp RAF.4. Numerous experiments were carried out on these aircraft, many of which remained at Farnborough.
The 14th machine saw service with the RNAS and carried serial no. 26.
Possibly only 5 machines went to the RFC, with serials 334, 335, 361, 380, and 382.
Capt. J A Liddell of 7sqn RFC was awarded the VC flying an RE5 on 31st July 1915. Wounded in combat near Bruges he lost consciousness but recovered in time to make a safe landing at Furnes. He subsequently died of his wounds, but I assume his action saved the life of his observer 2Lt RH Peck.
Jack Bruce records armament as a single Lewis gun, makeshift combinations of pistols and rifles, and a "small load" of 20lb Hales bombs.
So not a great aeroplane, but undoubtedly it contributed significantly to later developments.