Please refer to following answers of your queries:
Query 1. Is there any standard size for Images to Upload ?
Response : In the customizer, while uploading the images there the specific image size mentioned. Upload your images as per the image size. For thumbnail put images of size 600 × 600 pixels.
Query 2. What is the method to add Log In and Register Buttons?
Response : As per your query about adding the buttons, this feature is not available for now.
Query 3. How can I remove that blue background while adding Images and make a color of my own?
Response : For the specific identification please provide the screenshot and marking the place you want to remove the background color of.
Query 4. In the header sector of Demo ones, it worked 4 menus properly. But, when I am doing the same, 4 menus for the left hand section, last one comes in the second row. How can I fix this?
Response : Regarding this query, please provide your site URL for the fixes. This might have happened because of your logo size.
Hoping for your quick response.
If you have further query, let us know.