What scan resolution should we use for a large archive of 35mm color slides intended for web picture-library use?

Choose the scan resolution based on the smallest detail you need to preserve, not simply “maximum” for every slide. Scanning at the highest possible resolution gives you the most flexibility later, but it also greatly increases scan time, storage needs, and post-processing effort—important with 50,000 slides.

A practical approach is:

• If this archive may need future reuse, rescanning is difficult, or some images may need larger reproduction, scan as large as your workflow reasonably supports.
• If the files are mainly for web/picture-library preview and general digital use, scanning everything at maximum resolution is often unnecessary overkill.
• Use a resolution that captures the important image detail; beyond that, extra dpi mostly records more film grain and creates larger files without meaningful added information.

In short: don’t default blindly to the scanner’s maximum setting. Balance quality against storage, scan time, and labor. For an archive this large, it can make sense to use a sensible master-scan resolution that preserves the useful detail, then create smaller JPEGs for website delivery.