PlotLegends and ImageSize

This might help. Here I have a helper function to make the legend into an Inset:

plot = Plot[Evaluate[Exp[-4 f] /. {{f -> t}, {f -> t^2}}], {t, 0, 3}, 
   BaseStyle -> {FontSize -> 11, AbsoluteThickness[1.5]}, 
   PlotStyle -> {Blue, Red}, AxesLabel -> {"t"}, 
   PlotLabel -> "Negativity", 
   PlotLegends -> Placed[{"OU", "FBM"}, {.8, .5}], ImageSize -> 9 cm];

makeInset[plot_Legended] := 
 Show[{plot[[1]], 
   Graphics[
    Inset[plot[[2, 1, 1]], Scaled[plot[[2, 1, 2]]]]]}]

and then wrap plot with this function:

Export["Neg_long_OU_FBM.pdf", makeInset[plot]];