Would this be wide enough for a walker?

This is not wide enough for a walker.

What are the exact measurements. Width & length?

19 inches long, 15 inches wide, 3 inches tall. I use it to get into my tub easier, very stable.

Is this sturdy to use outside at my front door?

Yes, be sure it is on level ground or cement. I have one by my front door, one by the front step, and two more by the steps leading to the garage, and one by the step from the house to the back porch. I had knee surgery and they really help getting in and out of the house.

What is the exact weight of this?

Mine has been used outside as a half step at the front and back doors. I have had mine since Feb . 2017 and it weight today is 3.3 pounds and that is also weighted so they do not move.. They are L 19.5 X W 15 X H 3.5 I use 2 so it make the step as wide as the door. I have RA and osteoarthritis they work great. Even with high winds they DO NOT move. Great for elderly for I am also elderly.