Image size upload limit

Hi dear,

As my site is very slow and I don't know why I was thinking to limit the image size upload for every subsites by adding this limit in the network settings as shown in the attached Screenshot.

I am not sure if this setting works also for subsites and for marketpress product images.

However after I added that limit a subsite added a bigger pictures.

Can you please help me to understand how that network limit work and to add that limit for marketpress product