I made this quick summary of some of the size and limitations and specs for developing Amazon Alexa Skill. Please contact me at Amazon Skill Developer (Dallas, Texas) if you need one developed.

Max 8000 characters of speech when Alexa is speaking based on text.

MP3 – must be bit rate 48kbps, sample rate 16000 Hz, codec version (MPEG Version 2)
https://developer.amazon.com/docs/custom-skills/speech-synthesis-markup-language-ssml-reference.html#h3_converting_mp3

Maximum length of simple “conversational/speech” audio files is 90 seconds.
You can do long audio files of any length, such as podcasts, but that requires a separate programming model.

You can optionally include “cards” which a person can see at:
https://alexa.amazon.com/spa/index.html#cards

Brief explanation of cards here: https://developer.amazon.com/docs/custom-skills/include-a-card-in-your-skills-response.html

In addition to signing up with AWS, you also have to have a login here:
https://developer.amazon.com/edw/home.html#/.
There are two parts to the skill, the skill/vocabulary definition, and the software code.

Logo Images:
You need a small 108×108 PNG and a large 512×512 PNG file for getting certified and putting the skill in the skill store.

If you decide to use “cards”, see above, they have images as well, but the size requirements are different:

small 720w x 480h
large 1200x x 800h