There isn’t a single standard size for digital contact sheets. Historically, true contact sheets were made by placing film directly on photo paper, so each frame printed at the exact size of the negative. In digital, you choose the size based on what the sheet is for.

If the goal is image selection, make the thumbnails large enough to judge composition and obvious technical issues. If several images are very similar, printing them too small makes it hard to pick the best one. A practical guideline from the community answers is about 70 × 48 mm per image (roughly double a 35mm frame), which lets you fit about 12–16 images on an A4 or 8×10 sheet.

In practice, “big enough” means you can see what matters for your review. Professionals historically also used a loupe or magnifier with contact sheets, especially when the images were small. So choose a size that suits your project and viewing method rather than aiming for one universal professional standard.