When you export in Flexitive, you will have the option to Export 2x Size Images in the export panel.
Toggling this setting on will export all images assets for your export to have twice the standard resolution. For example, if an image will normally be exported with a resolution of 300x250, it will instead export with a resolution of 600x500.
Exporting 2x Size Images only affects your design if you are selecting Export Creative (.ZIP) or if you have the Only Export Images option selected. Tags exported with Export 2x Size Images selected will not be affected by this option.