After Picasa shut down and the picture storage for Blogger was replaced with Google Drive, there seems no information on this limitation, so I tried to figure it out using panoramic photos I took.
Note: I choose my Google Photos' upload option "High quality (free unlimited storage)" which lets Google resize bigger images.
Before uploaded: 4206*1577 px (2.2 MB) / After uploaded: 2896*1086 px (877 KB)
Before uploaded: 5970*1194 px (1.4 MB) / After uploaded: 3965*793 px (528 KB)
Before uploaded: 2406*1256 px (1.4 MB) / After uploaded: 2048*1069 px (778 KB)
Before uploaded: 2304*1137 px (493 KB) / After uploaded: no change (482 KB)
Before uploaded: 2712*1356 px (882 KB) / After uploaded: 2508*1254 px (566 KB)
Before uploaded: 3816*1272 px (927 KB) / After uploaded: 3072*1024 px (486 KB)
With single exception, the photos which got resized have now approximately 3.14 megapixels. The only unchanged photo was smaller than that threshold before uploaded. Based on these result, you can expect that images having less than 3.14 MP will pass resized.
To verify this I made another panorama with dimension of 2800*1120 px, just a hair under the threshold:
Before uploaded: 2800*1120 px (627 KB) / After uploaded: no change (612 KB)