I've taken note of the key phrase you provided: 'Mozilla Thunderbird 115.3.1 download the new version for apple'. Based on your text, I understand that you're looking for information about the latest version of Thunderbird email client.

Here are some key points and answers to your questions:

* Thunderbird is a free and open-source email desktop client developed by Mozilla.

* The latest version available is 115.3.1, which has several updates and improvements over previous versions.

* Apple users can download the new version for their devices.

* To change the default text size in Thunderbird Version 102.10.1, you have multiple options to adjust the font size.

Regarding sending large attachments:

* Thunderbird has a limit on attachment sizes, but you can increase it by adjusting the settings. Here's how: Go to 'Tools' > 'Account Settings' > 'Server Settings', and then look for the 'Attachment Size Limit' option. You can change this setting to allow larger attachments.

If you're experiencing issues with small text size and bunched up messages in Thunderbird 115.3.1, try adjusting the font size settings as mentioned earlier. If the issue persists, it might be a bug that needs to be reported or fixed by Mozilla developers.

Lastly, note that the notes provided apply to Thunderbird version 115 beta 6 released on June 29, 2023, and upcoming release SuperNova UI features an updated user interface with new design elements.

Please let me know if there's anything else I can help you with!