I checked some infant patterns and each said 3/4 yd of 1/2 inch elastic for the different views. The patterns showed separates with elastic at the ankles and at the waist. I think that you would be safe using that amount. If you could get the correct size from the parents, then that would be better. I would use about 1/2 yd for the waist and 1/8 yd for each ankle. When a pattern calls for elastic, I always made a band as I don't like elastic at the upper arm, waist or ankles on babies. Hope this helps you figure out what you need.
Marty
Brighten each corner where you are ... smile!