if (MarkerShapeDefault != MarkerShape.None)
{
    item.MarkerStyle.Shape = item.MarkerShape != MarkerShape.None ? item.MarkerShape : MarkerShapeDefault;

    // NOTE: tried this but size seems to be defaulting something to something small, 
    // so you can barely see the shape, defaulting to font size for now
    //item.MarkerStyle.Size = item.MarkerStyle.Size != 0 ? item.MarkerStyle.Size : item.LabelFontSize;
    item.MarkerStyle.Size = item.LabelFontSize;
}
else
{
    item.MarkerStyle.Shape = item.MarkerShape;

    // If the marker size is not set, use the label font size as a fallback
    // NOTE: Same as above, tried it, but somehow the default is really small, 
    // setting to font size for now.
    //item.MarkerStyle.Size = item.MarkerStyle.Shape != MarkerShape.None && item.MarkerStyle.Size != 0 
    //    ? item.MarkerStyle.Size : item.LabelFontSize;

    item.MarkerStyle.Size = item.LabelFontSize;

    if (item.MarkerShape == MarkerShape.None && item.MarkerStyle.Shape == MarkerShape.None)
    {
        item.LineStyle.Render(canvas, symbolLine, paint);
    }

    item.FillStyle.Render(canvas, symbolFillRect, paint);
    item.OutlineStyle.Render(canvas, symbolFillOutlineRect, paint);
}

// 1) Add your plottables
var xs = Generate.Consecutive(51);
var ys1 = Generate.Sin(51);
var ys2 = Generate.Cos(51);

var sp1 = formsPlot1.Plot.Add.Scatter(xs, ys1);
sp1.LegendText = "Sine";
sp1.MarkerSize = 15;
sp1.LineWidth = 0;
sp1.Color = Colors.Magenta;

var sp2 = formsPlot1.Plot.Add.Scatter(xs, ys2);
sp2.LegendText = "Cosine";
sp2.MarkerSize = 45;                       // big marker
sp2.MarkerShape = MarkerShape.OpenSquare;
sp2.LineWidth = 0;
sp2.Color = Colors.Green;

var legend = formsPlot1.Plot.ShowLegend(
Alignment.LowerRight,
 ScottPlot.Orientation.Horizontal);

// make the square each symbol sits in ≥ the largest marker
 float big = formsPlot1.Plot.PlottableList
.OfType<IHasMarker>()
.Max(p => p.MarkerStyle.Size);

legend.SymbolWidth = big + 6;

formsPlot1.Refresh();