Is it possible to upload an app that is ~800 megabytes in size to an App Center tenant?
There is currently no explicitly defined size limitation on apps in App Center, though discretion should be exercised for app sizes due to impacts they may have. For example, it would not be advisable to upload an app that is multiple gigabytes in size as it would impact the database, available file system space, or licensed space (applicable to a SaaS based tenant).
When attempting to upload a large app, a timeout may occur due to the web server's configuration. To avoid timeouts, a second internet browser tab may be opened and continuously "refreshed", or different App Center pages loaded, while the app is being uploaded in the initial internet browser tab. This workaround acts as a "keep alive" to prevent the web server from timing out the session. It is also recommended to upload large apps while physically connected to a local area network (LAN), as opposed to Wi-Fi, and to avoid uploads over a VPN connection.
It is a good idea to keep in mind that for an application to be approved on Apple's App Store it is required that they be under 2 GB in size. Before testing the upload of an in-house app to the App Center we recommend making the attempt to sync the app through iTunes on an iOS device. If this is successful, the app should have the ability of being uploaded through the Admin Console without failure.
Refer to TECH188525 for an elaboration on how iOS handles wireless requests for .ipa downloads and installations.