BLE 5.0 MTU max byte size.

If your peripheral supports Data Packet Length Extension, it must perform the packet length update procedure before performing the Exchange MTU Request handshake. This will allow the Apple device to optimize the negotiated MTU accordingly.

Data Packet Length Extension is an enhancement introduced in the Bluetooth 4.2 specification which increases the maximum data length from 27 to 251. Once your device completes the DPLE process successfully then the MTU will be negotiated.

When operating as a central, the Apple device will negotiate the optimal MTU size for that point in time, based on factors such as the Bluetooth topology, connection event length, maximum data length, and protocol (GATT or connection-oriented L2CAP). You cannot force a certain MTU from your peripheral, or from within your app.

Depending on your peripheral it might be possible to get even 512 bytes if negotiated. This is possible starting from Iphone 7S.

On mac OS this is different because the onboard bluetooth chip set does only support it when the macbook is after 2019. There are dongles like the Laird BT851. This can be used with older mac books. Although it does show support of 4.2 it only allows 27 bytes, since the dongle is not on apples internal DPLE list. This list is somehow hidden and I did not figure out how to put a dongle on this list nor the support was yet able to tell me what enables a dongle to get on the list.

I tried Iphone6, it's only can receive185 bytes by BLE5.0 and can't negotiate the MTU size. I tried Iphone7s too. It can receive bytes more than 185 and can negotiate the MTU size. So, i agree with the second answer.