Transformers and enclosures are the most expensive parts in hobby amp design, so it's a good idea to try a few things out and read up about amp power requirements before you go further in this hobby. There are two schools of thought here:
1. Audiophiles/perfectionists like to feed a sinusoid into their Class-A amp, connect it to 4 (or even 2) ohm loads, drive it at full power, and watch the output signal for clipping. If there is clipping, they will throw more Transformer and capacitors at it!
2. Engineers tend to go with class-d or class AB(with a low bias) or chip-amps (for lower powered commercial products). They also know that people generally feed their amps with music which has a far smaller duty cycle than a sinusoid. They also know that it is more reasonable to plan for the average listening experience (i.e. not a full blast) unless they are designing the amp for outdoor rock-concerts.
Here is my philosophy:
1. find out what load you want the amp to work with (8, 4 or 2 ohms). Look at the SOA of your output devices, and find out how much power they can deliver at working temperature. Assume .2 to .1 duty cycle since you are playing music and not sinusoids (or 0.5 if it's a subwoofer amp). Make sure you know what speakers you will be using with this. If you are using them with a pair of 25 Watt speakers e.g., and you listen at barely 1/10 volume, there is no reason to buy a huge transformer - you can always limit the input signal to avoid clipping.
2. Always add proper fuses in the primary AND secondary of your supply. Get the current/Voltage rating for the fuses perfectly right. Then add thermal cutouts (about 65 degC) and bolt them to sinks and transformer. You want to be safe - nothing puts you off in this hobby like a fire in your house - wifey tends not to approve of those.
3. For amps similar to your design (2 channels each with two pairs of output MOSFETS, into an 8 ohm load) I have gotten away with anything from 150 VA (for home use with studio speakers) to 750VA for 4Ohm outdoor party speaker boxes at full blast. Yes the thermals might kick in now and then, but that's a good thing - it will stop you from going deaf!
4. with the cutouts and proper fusing in each rail and the input I have yet to have a transformer blow on me. Additionally, since we are keeping the cost of the hobby low by being reasonable with Transformer and Capacitor selection, you will have some money to spend on the most important part of the amp: the enclosure! Believe me you will get a lot of enjoyment out of a small good looking amp, and none from a circuit board in a giant crusty old box.