What’s in the deck

54 cards in three suits of 18: Length (how long), Mass (how heavy) and Height (how tall). Each card shows a thing, exactly what is measured, and its value, printed small and upside down at the foot so nobody reads it by accident. Turn the card round to read it.

Only compare cards of the same suit. Length meets length, never mass. Inside a suit every pair is a fair duel: the bigger one is at least 15% bigger, and for animals and other things that vary, the typical ranges never overlap.

Get ready

1. Cut out the cards (and glue or print the backs on, if you like).
2. Sort them into three suit piles, shuffle each pile and put it face down.
3. Have a pencil and paper ready to keep score.

Play

1. On your turn, pick a suit pile and turn its top two cards face up, side by side. Read out the names and what is measured.
2. Everyone counts “three, two, one” and points at the card they think is bigger: longer, heavier or taller.
3. Turn both cards round and read the values. Everyone who pointed at the bigger card scores a point.
4. Put the pair aside. The player on the left goes next.

The first player to 10 points wins. In a long game, play until the piles run out.

Higher or lower (1 or more players)

Turn up one card. Before you turn the next card of the same pile, say “bigger” or “smaller”. Keep going until you are wrong; your score is the number you got right. Beat your best.

Line them up (2–4 players or teams)

Deal five cards from one suit to each player, face up. Put them in order, smallest to biggest, without turning them round. Then check the values: one point for every card in the right place.

Classroom: stand left or stand right

Hold up two cards of one suit. Pupils go to the left or the right side of the room (or hold up one or two fingers) for the bigger one. Reveal, then ask the group what surprised them. Scan a card’s QR code to show the two things drawn to the same scale on a screen.

The QR codes

Each code opens that thing’s page on Size Duel, with its drawing and where the number comes from. Scanning is optional: the game works without a phone.

What pupils practise

• Measurement and units: micrometres to kilometres, grams to tonnes, with imperial units in brackets.
• Comparing and ordering lengths, masses and heights, and saying what exactly is measured (“nose to tail”, “height of the outer wall”).
• Estimation: pupils commit to a guess before they see the number.
• Ratios: “how many times longer?” Divide one value by the other.
• Powers of ten and standard form: the Mass suit goes from a table tennis ball (2.7 g) to the Sun (1.99 × 10³⁰ kg).

Suggested lesson (20–30 minutes)

1. Show one pair of cards and vote. Reveal and talk about why most people guessed what they did.
2. Play in groups of three to five for ten minutes.
3. Give each group one suit to line up from smallest to biggest, then check against the answer key.
4. Ask each group for the pair that surprised them most, and why.

Questions to ask

• Why can’t we compare a length card with a mass card?
• Why do some values say “about”? What would make a fair comparison between two animals?
• A blue whale is about 25 m long. How many giant anteaters is that, nose to tail?
• Which is easier to picture: 828 m or 2,717 ft? Why do we use both?

Where the numbers come from

Every value is from Size Duel’s size deck, checked against two independent sources (often Wikidata plus an encyclopedia, a museum, a national park service or an official rule book). Each card’s QR code opens its page with the sources. Animals use typical adult values; their natural range is kept apart from every other card in the suit, so a big or small individual never changes an answer.

Printing

Print at 100% (“Actual size”), not “Fit to page”. Thin card (160–200 g/m²) works best; on ordinary paper the values can show through. For backs, print the double-sided file and flip on the long edge, or print the backs page on the reverse of each card sheet. Cut along the grey lines, or use the corner marks with a ruler and craft knife.

The cards print well in black and white: every suit is named in words and has its own symbol (a ruler, a weight, an arrow up).