I assume you mean MR 29.7.1. No IOS here.
Firmware image sizes will vary and we don't publish the sizes. Each AP will download the firmware individually. You can choose from two firmware upgrade strategies to lessen the impact to your wireless clients https://documentation.meraki.com/MR/Other_Topics/Access_Point_Firmware_Upgrade_Strategy.
Ryan / Meraki SE
If you found this post helpful, please give it kudos. If my answer solved your problem, click "Accept as Solution" so that others can benefit from it.