There are multiple issues here. I don't know what's going on or what's causing them so I'll just attach a few renderings, briefly explain how I obtained them, and hope the (unintentional?) differences between 5.2.0 and 5.3.0-RC1 speak for themselves.

In a script(*), I render a scene to a PNG file. I get a render view and set its size like this

renderView1 = GetActiveViewOrCreate('RenderView')
renderView1.ViewSize = [1000, 400]

I also attach a color bar to the plot

velocityLUTColorBar = GetScalarBar(velocityLUT, renderView1)
velocityLUTColorBar.Title = 'Velocity'
velocityLUTColorBar.ComponentTitle = 'Magnitude [mm/s]'
velocityLUTColorBar.AutomaticLabelFormat = 0
velocityLUTColorBar.LabelFormat = '%.1f'
velocityLUTColorBar.RangeLabelFormat = '%.1f'
velocityLUTColorBar.NumberOfLabels = 6
velocityLUTColorBar.TitleFontSize = 7
velocityLUTColorBar.LabelFontSize = 6

velocityLUTColorBar.TitleJustification = 'Centered'
velocityLUTColorBar.AutoOrient = 0
velocityLUTColorBar.Orientation = 'Horizontal'
velocityLUTColorBar.AspectRatio = 40.0

# I do not understand these numbers
velocityLUTColorBar.Position = [0.36, 0.75]
velocityLUTColorBar.Position2 = [0.3, 0.0]

and put an annotation in place (this rendering is really one frame from a video(**)):

pythonAnnotation2 = PythonAnnotation(Input=timeInfo)
pythonAnnotation2.ArrayAssociation = 'Row Data'
pythonAnnotation2.Expression = """
'simulation time: {:.3f}s'.format((relativeTime[0]) * 1000)
"""
pythonAnnotation2Display = Show(pythonAnnotation2, renderView1)
pythonAnnotation2Display.WindowLocation = 'AnyLocation'
pythonAnnotation2Display.Position = [0.07, 0.05]

Now I observe four differences in the output between 5.3.0-RC1 and 5.2.0, all of which I find undesirable:

1. With 5.3.0-RC1, I get a 500x200 PNG instead of the 1000x400 I would expect (and which I get with 5.2.0!)
2. (After requesting 2000x800 with 5.3.0-RC1, resulting in a 1000x400 PNG) It seems font sizes are slightly different now (smaller)
3. (with the aforementioned change still) There are no periods on the colorbar tick labels
4. (with the aforementioned change still) If you look at the annotation at the bottom, you can see a nonsmooth transition from the upper half to the lower half, most noticeable in the numbers 7 and 9.

To illustrated, I'm attaching three PNGs: (1) The 1000x400 with 5.2.0, (2) the 500x200 with 5.3.0-RC1 arising from the same script and (3) the 1000x400 with 5.3.0-RC1 resulting from a hack to the script.

(*) I don't mind sharing the script but it needs to load some data and so I expect it would be of limited use. I have not worked on a minimal working example, yet, hoping that one will not be necessary. (**) The complete video is supposed to look like this one obtained with 5.1.2.

PS: I've manually disabled FXAA through renderView1.UseFXAA = 0.