The query "how big is a" is ambiguous, but based on common interpretations, it might relate to standard sizes such as paper sizes categorized under the A series (like A0, A1, A2, A3, A4, etc.). The A series paper sizes are internationally standardized under ISO 216 and use an aspect ratio of 1:√2. Here are the dimensions of several common sizes in the A series:

• A0: 841 × 1189 mm (33.1 × 46.8 inches), the largest standard size, with an area of 1 square meter.
• A1: 594 × 841 mm (23.4 × 33.1 inches)
• A2: 420 × 594 mm (16.5 × 23.4 inches)
• A3: 297 × 420 mm (11.7 × 16.5 inches)
• A4: 210 × 297 mm (8.3 × 11.7 inches), the most commonly used size for letters and documents.
• Sizes go progressively smaller down to A10, each size being half of the previous size when folded along the larger dimension.

If the intent was different, such as "how big is an animal," "how big is a building," or something else starting with "a," please clarify for a more precise answer. For now, the size of an "A" paper, specifically A0, is 841 × 1189 mm or 33.1 × 46.8 inches.