Learn Turkish: Akvaryumda büyükçe bir salyangoz var. - There is a largish snail in the aquarium.

It acts as a softening or approximation suffix. While büyük means "big" or "large", adding -çe turns it into büyükçe, meaning "largish", "fairly big", or "on the larger side". It tells us the snail is relatively big, but tones down the absolute strength of the adjective.

Why is the suffix spelled -çe here instead of -ce or -ca?

This happens due to two basic pronunciation rules you learned earlier. First, consonant mutation: büyük ends in k, which is a voiceless (hard) consonant (remember the Fıstıkçı Şahap rule). This causes the c in the suffix to harden into a ç. Second, vowel harmony: the last vowel in büyük is ü, which requires an e according to 2-way vowel harmony. Together, they give us -çe.

Wait, doesn't the -çe / -ca suffix usually create language names, like Türkçe?

Great connection! It is exactly the same suffix, but its function changes depending on the type of word it attaches to. When attached to a noun indicating a people or nationality (like Türk or İngiliz), it means "the language of" (Türkçe, İngilizce). When attached to an adjective (like büyük), it acts as an approximation meaning "-ish" or "fairly" (büyükçe).