Do you mean a pair of these ?
In which case you will need 8mm internal diameter pipe (the width across the peaks on the connector stub is actually 9mm.
Bolted-up pipe clips are required as the fuel line runs at (from memory) 34 psi.
BUT . . Be aware that the standard BMW plastic fuel line connectors are considered suspect in some quarters.
It seems that the 'O' rings can swell when petrol reaches them, resulting in the plactic connectors breaking.
It has also been reported that the plastic components can go brittle with age and fracture without any help from the 'O' rings.
In the USA, BMW did a recall on the K1200 RS / GT models because of a number of incidents.
It seems that BMW advise Workshop Technicians to replace the 'O' rings at standard service intervals.
According to two web sites I have looked at, the 'cure' is to use metal connectors with 'VITON' 'O' rings that do not degrade in petrol.
I have just ordered two sets (one for my 2004 1150GSA, the other for my 2003 K1200GT) from Omega.co.uk.
The eight connectors, carriage & VAT cost £111.27.
Not cheap but possibly worthwhile insurance as I expect to be in the north of Norway around mid-2008, followed by a trip to Bosnia (and all places in between it and Scotland) around September 2008.
Its one of those decisions that may save hassle or not - one will never know.
I'll provide the connector details and the web site links if you want.
Bob.