Microsoft currently on the Insider preview ISO page offers build 15058 ISO’s, but unofficially ISO’s have been made live though Dona Sarkar, chief of Windows Insider program promised ISOs later this week.
@itsmichaelwest yes, it will be about a week
— Dona Sarkar (@donasarkar) March 23, 2017
Downloading Windows Insider preview ISO’s build 15063
The size of 32 bit ISO is 3.0 GB and of 64 bit ISO is 4.1 GB.
So from now on, Windows 10 15063 would only get cumulative updates as there is no doubt which is RTM. If you want to get the Windows 10 creators update via official channel, you need to wait till 11th April which is widely speculated date for Creators update.