We could wrap the icons which are smaller than the considered area so that we can prevent showing low-resolution images. It also shows the real size of the smaller icons.

The wrapper might be a simple rectangle (Perhaps having a short border-radius) and could have a different color close to the background color tune.

The icons which are larger than the considered size might be rescaled to the maximum size limit while keeping the aspect ratio.

The wrapper is only a quick suggestion, and probably a better solution might be considered.