There isn’t one universal standard. A good online portfolio size depends on your goals: display quality, your audience, and how much you want to discourage copying.

For typical web viewing, many photographers resize so the long edge is roughly 900–950 pixels. That fits well on common screens while still looking reasonably large. If you want a more immersive presentation, something around 900–1400 pixels on the long side is often used.

If theft is a major concern, some photographers go much smaller, around 600 pixels, though that limits visible detail. In that case, or when showing larger proofs to serious clients, it can help to use private galleries or restricted-access previews instead of posting full-size images publicly.

So, a practical approach is:

• public portfolio display: about 900–950 px on the long edge
• larger showcase images: up to about 1400 px on the long edge
• theft-sensitive previews: around 600 px, possibly with a watermark

Avoid posting full 3000×5000 files publicly unless you specifically want viewers to access that level of detail.