is it correct to say 'will be agreed
It will be agreed on a specific size for those images so that developed in large size, and the system will coordinate them by three or four sizes used on the site and determine their directions.

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It's actually really confusing.

How about, "A specific size will be agreed upon..."? (We need the subject to be clear.) Still, the rest of the paragraph needs revision.

Evden çıkmadan dil öğrenme fırsatını kaçırmayın. Deneyimli dil eğitmenlerimizden oluşan seçkimize göz atın ve ilk dersinize şimdi kaydolun!