i,ve noticed that all 6wt lines are not the same diameter . would i be right in thinking that a finer line will give a finer presentation although two lines were the same weight. If so I wondering if there is a line that,s known to be fine for it,s weight and also have a good name. or would a line that has a longer taper be the answer ?

Cortland Sylk lines are 'thin' to the extent that there is little visible taper compared to say a short headed line such as Snowbee XS. However, what we're talking about here are differences that are so small that line diameter probably has little impact upon presentation compared to overall taper / profile of the front 30ft or so.

Let's also remember that because (floating) lines are made from materials that, obviously, must float manufacturers are bound by the laws of physics and the limits of materials available.

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I think you'll find, at the risk of opening an old debate, that some members on here don't believe that the laws of physics apply to fly lines.

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Hi', Scooby.
Generally, the 6wt floater will have the greatest diameter. I'm not sure how the modern floaters get their buoyancy, but they used to be made like Aero chocolate -- filled with tiny bubbles. That means bulk.
By comparison, you could expect all 6wt sinkers of the same make to be thinner.
A point that used to intrigue me was that, at one time, even the manufacturers admitted that a 7wt line would sink a little faster than a 6wt of the same make and same nominal sinking rate. That is 7wt Inter would sink faster than a 6wt inter of the same manufacture.
Things have, or should have, moved on since then. TC

the length and speed of the taper determines the presentation rather than the diameter, a short taper dissipates energy more quickly than a longer one, long gradual tapers dissipate energy more gradually.