I generally save the image as a jpeg...then cut and paste the image into MS Word. Just so I can see it on an 8.5x11 sheet and move it around to add additional images if needed.
Here are some sizes that I typically use when making things...but really the best thing to do is remove the label from the item (water bottle, candy bar, etc.) and then measure it to get the correct dimensions.
Every water bottle is different but typically 8.5x2 inches fit most water bottles.
I'm not sure what you mean by large candy bar wrappers...I regular milk chocolate hershey bar wrapper measures 5.25x5.5 inches. But every candy bar is different. If you get a hersheys chocolate bar with almonds it is smaller. So if you aren't getting a regular milk chocolate hershey bar...remove the wrapper and measure it.
For the aspirin pouch...what you are putting in it and whose design you are using will determine the size. Some people put two aspirin in it but most put the individual packets of aspirin in the pouch. If you are going to use the individual packets of aspirin...measure it then use the ruler on the top and side of word to see if the front of your pouch will be big enough. Hope that makes sense.