What is the ideal size for Main image?

Just use a "normal size" photo for main theme.

General rule for main theme image size:
Try not to use main images that are less then 5 times bigger in pixels than size of mosaic in tiles.

Let's suppose that you want to make mosaic with 30 columns and 40 rows:

30 x 5 = 150
40 x 5 = 200

I.e. any picture starting with as small as 150 x 200 pixels and bigger is suitable for the Main image. Simply select an image where all essential details are clear visible. There are no special restrictions on picture size.

Probably you have a much bigger photo. It's OK! Use 5 - 10 - 20 Mega Pixel shots as main photo. No problem! You don't need  to downsampling nor upsampling your Main image. Just open it with Mazaika and mosaic it.

Mazaika - (C) 2000-2018 by Boris A. Glazer.  www.mazaika.com