There are a couple ways. The easiest way is to open up your render in Paint or Photoshop and reduce the dimensions (resolution) by resizing. Then go to "File" > "Save As", then under "Save as type" for sharing on the web you should save in jpeg format which has a smaller file size.
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Daz3d can export in a number of formats such as PNG or JPG (jpeg). PNGs are good for higher quality but are much larger in size. Jpegs are much smaller but lose quality. Most of the time though, no one can tell the difference. If you're not concerned, jpeg is a good way to go.
Resolution matters. If you're just sharing something with friends, somewhere between 500 or 1000 pixels on the smallest side of your image is fine (for example, 1920x1080 for a "full size" image). Anything larger is mostly only necessary for special purposes or professional work.