title: How to specify marker size as diameter in input space with plotly Scatter3d?
body: <p>Experimenting with go.Scatter3d I noticed the option marker sizemode="diameter". It seems that this option is not related to the diameter of the markers in input space. </p>
  
  <p>Is it possible to resize the marker with custom diameter in input space? </p>
  
  <p>E.g. I want markers, each with a diameter of 5 in terms of the input space:</p>
  
  <pre><code>import plotly.express as px
  import plotly.graph_objects as go
  
  import numpy as np
  
  x =y=z = np.arange(1,4)
  
  fig = go.Figure()
  
  fig.add_trace(
      go.Scatter3d(
          mode='markers',
          x=x,
          y=y,
          z=z,
          marker=dict(
              color=px.colors.qualitative.D3,
              size=5,
              sizemode='diameter'
          )
      )
  )
  axis_range = [-5,10]
  fig.update_layout(scene = dict(
          xaxis = dict(range=axis_range,),
          yaxis = dict(range=axis_range,),
          zaxis = dict(range=axis_range,)
  ))
  
  fig.show()
  </code></pre>