question

In Photoshop and similar programs, the file size you see in the "Image Size" dialog is uncompressed. Once you save as PNG, that file will usually be smaller. So export the PNG and look at the file size on your hard drive. PNG can get very very large if you have a lot of smooth transitions of color (rainbows, photos, etc.). In Photoshop, you can use some sort of filter or effect (try Posterize at 128 levels) to clump similar pixels together. When you save the PNG, it should offer to save it Smaller or Faster. Choose Smaller.

Let us know what app you're using; someone will be able to give you specific instructions.

where are you making your designs through?

Make your template and save it. Make your image the size you want. Copy and paste your image into the template. Adjust the placement or size of the image. Name your new image in the template, that way you will still have your blank template to use again and again.