Size presets

size in the specification, --size on the command line and the size option of the MCP tool chartlet_render pick a canvas for a destination:

chartlet render spec.json --size slide-16x9 -o slide.svg
chartlet render spec.json --size print-a4-landscape -o page.svg
chartlet render spec.json --size slide-16x9 --format png -o slide.png

A preset is more than a viewBox. The chart is laid out at the canvas divided by the preset’s scale and drawn that much larger, so type, strokes, margins, label wrapping, legend rows and the minimum geometry of a diagram are decided at the size they are read at: a long title wraps and a legend breaks into rows for the room the canvas gives, not for the document size.

Preset	Canvas (CSS pixels)	Scale	Layout size	Colors
doc	800 × 450	1	800 × 450	custom properties
doc-wide	1200 × 500	1	1200 × 500	custom properties
slide-16x9	1920 × 1080	1.6	1200 × 675	custom properties
slide-4x3	1600 × 1200	1.6	1000 × 750	custom properties
social-og	1200 × 630	1.25	960 × 504	custom properties
social-square	1080 × 1080	1.2	900 × 900	custom properties
print-a4-landscape	1123 × 794	1	1123 × 794	literal
print-a3-landscape	1587 × 1123	1	1587 × 1123	literal
print-letter-landscape	1056 × 816	1	1056 × 816	literal

The print canvases are the paper at 96 dpi, so the document type size (12 px labels) is 9 pt on paper. On a slide the 12 px label becomes about 19 px at 1920 pixels wide, and the text of a diagram grows by the same factor. The SVG has the canvas as width and height and the layout size as viewBox, so a page that scales the SVG scales all of it together.

Precedence

From the strongest to the weakest:

1. The mobile and social variants (--variant mobile, --variant social) ignore size: they have a size of their own.
2. An explicit width or height in the specification overrides that side of the canvas. The scale of the preset still applies, so "size": "slide-16x9", "width": 1000 is 1000 × 1080 pixels laid out at 625 × 675.
3. size (the specification’s, or --size, which replaces it).
4. The default size of 800 × 450 without a size.

The print variant (--variant print) with a size draws the preset’s canvas with literal colors. A print preset in the default variant does the same for SVG, with the IDs of the print variant (ending in -p) and no hooks; the HTML format ignores the literal colors and keeps the custom properties. PNG always rasterizes with literal colors and the bundled Inter font (see Social images and PNG), so a PNG of a preset has exactly the preset’s pixels at --scale 1.

Without size, or with doc or doc-wide, the output is byte-identical to a specification without it.

When the content does not fit

Nothing is clipped and nothing is dropped. A chart is laid out at exactly the layout size and shortens or wraps labels with their usual warnings (text_truncated, labels_thinned, value_labels_omitted). A diagram that needs more room than the canvas keeps the room it needs: the SVG is larger than the preset and the warning size_exceeded at /size names the size it needs. Shorten labels, remove elements, split the diagram, or choose a larger preset.

diagram_over_budget uses the layout size, so a slide reads the same number of steps as a document of that layout size, with larger type.

Rust, JavaScript and MCP

size is a field of the specification, so every API takes it. In the Rust API a specification read with ChartSpec::from_json resolves it; SizePreset::dimensions, scale and is_print describe a preset. chartlet_render takes a size option that sets the specification’s size. The schema lists the names in size.