2 added 2 characters in body
source | link

If you remove Joined->FalseJoined->False it seems that you can get a better length for the given line thickness:

Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, 
PlotLegends -> 
LineLegend[Automatic, 
LabelStyle -> Directive[FontFamily -> "Helvetica", Medium], 
LegendMarkers -> 
Graphics[{Thickness[0.2], Line[{{-10, 0}, {10, 0}}]}]], 
PlotStyle -> Thickness[0.005], AxesStyle -> Thickness[0.005]]

thicker and longer lines

If you remove Joined->False it seems that you can get a better length for the given line thickness:

Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, 
PlotLegends -> 
LineLegend[Automatic, 
LabelStyle -> Directive[FontFamily -> "Helvetica", Medium], 
LegendMarkers -> 
Graphics[{Thickness[0.2], Line[{{-10, 0}, {10, 0}}]}]], 
PlotStyle -> Thickness[0.005], AxesStyle -> Thickness[0.005]]

thicker and longer lines

If you remove Joined->False it seems that you can get a better length for the given line thickness:

Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, 
PlotLegends -> 
LineLegend[Automatic, 
LabelStyle -> Directive[FontFamily -> "Helvetica", Medium], 
LegendMarkers -> 
Graphics[{Thickness[0.2], Line[{{-10, 0}, {10, 0}}]}]], 
PlotStyle -> Thickness[0.005], AxesStyle -> Thickness[0.005]]

thicker and longer lines

1
source | link

If you remove Joined->False it seems that you can get a better length for the given line thickness:

Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, 
PlotLegends -> 
LineLegend[Automatic, 
LabelStyle -> Directive[FontFamily -> "Helvetica", Medium], 
LegendMarkers -> 
Graphics[{Thickness[0.2], Line[{{-10, 0}, {10, 0}}]}]], 
PlotStyle -> Thickness[0.005], AxesStyle -> Thickness[0.005]]

thicker and longer lines