Setting the size of mark_rect

My question is regarding setting the size (width and height) of each of the cells in mark_rect(). How can we do that such that the size is fixed, and the overall heatmap just gets bigger if we have more boxes to show?

[jakevdp]

You can do this with mark_square() - this is akin to mark_point() and mark_circle() in that it generates points that can have a size encoding, whereas rect generates rectangles whose size is controlled by x,x2,y,y2 encodings:

import altair as alt
import numpy as np
import pandas as pd

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-10, 10), range(-5, 5))
z = x ** 2 + y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                     'y': y.ravel(),
                     'z': z.ravel()})

alt.Chart(source).mark_square().encode(
    x='x:O',
    y='y:O',
    color='z:Q',
    size='z:Q'
)

[RoyalTS]

import altair as alt
import numpy as np
import pandas as pd

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-10, 10), range(-5, 5))
z = x ** 2 + y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                     'y': y.ravel(),
                     'z': z.ravel()})

alt.Chart(source).mark_rect(size=5).encode(
    x='x:O',
    y='y:O',
    color='z:Q'
).configure_scale(
    rangeStep=10
)

This seems to have stopped working. I get

SchemaValidationError: Invalid specification

        altair.vegalite.v4.schema.core.ScaleConfig, validating 'additionalProperties'

        Additional properties are not allowed ('rangeStep' was unexpected)