Change General preferences in Messages on Mac
In the Messages app on your Mac, use General preferences to change general options for using Messages.
To change these preferences, choose Messages > Preferences, then click General.
Choose how long to keep message conversations in your account. For example, if you choose 30 Days, your conversations (and all attachments) are available in Messages for 30 days. After that, they are automatically removed.
Save history when conversations are closed
Select to save your conversations when you close them. If you don’t select this option, closing conversations deletes them. Quitting Messages or closing the Messages window doesn’t close individual conversations.
Notify me about messages from unknown contacts
Select to receive a notification whenever anyone contacts you. If you don’t select this option, you get a notification only if you’re contacted by someone who is listed in Contacts or is someone you previously received messages from.
Notify me when my name is mentioned
Select to receive a notification whenever your name is mentioned in a conversation.
Play sound effects
Play the default sound effect for when any event occurs. For more details, see Play sound effects.
Message received sound
Choose the sound that plays when a message is received.
Save received files to
Choose where you want to store files sent to you by friends in Messages.
Move the slider to adjust the size of text in Messages.