You changed this CSS ? .attribute-squares .attribute-square { display: table !important; width: 32px !important; height: 32px !important; border: 1px solid #444 !important; cursor: pointer; } You will need to clear cache and you browser cache and check that it is updated
If that does not work the file to look at is src\Presentation\Nop.Web\Views\Product\_ProductAttributes.cshtml
But looks like there is a setting for the size used in the Factory _mediaSettings.ImageSquarePictureSize