Given your criteria, the shape, surface, and color of the shield does matter.
The trick is, once shielded from fuel and radiated energy, you want to keep hot AIR away from the target, not the fuel (unless the fuel actually sticks to the shield).
A white or silvery shield would be most effective in deflecting radiated heat away. A black shield would be the worst-case scenario. A slippery shield would prevent the fuel from sticking. Think about police shields, made of composite material, that deflect Molotov cocktails and the burning fuel from them.
The applied heat is not stationary (like a match), it is moving. If you keep the heat moving past the shield, it minimizes the effect. If the shield were appropriately shaped, it would deflect the air flow (and thus the heat energy) around it. I would suggest that a curved narrow shield would be best to deflect the air flow, and thus the majority of the heat. Think a branch moving across a fire vs the same branch stationary over the fire.
And the size and shape of the directed flame is important. Ever try to light a log with a match? But use several matches over a larger surface, you might do it. It is the amount of total heat that matters. A narrow flame jet is not as effective as the same jet with a wider circumference of flame. The total heat energy would be greater. A blow torch capable of burning through metal at close range would not be effective at 20 feet, but a jet engine would. Even a 50,000 BTU furnace burner will not ignite wood at twenty feet. It doesn't have enough force to carry the heat that far. Mind you, an unprotected human would get mighty uncomfortable from just the radiated heat. Remember, water boils at a lower temperature than wood ignites (you can't cause a wood fire by just pouring boiling water at atmospheric pressure over it, no matter HOW much water you use).
The fuel is important. How quickly does it burn? Over 20 feet, will it all have burned up, or will there still be fuel left to burn, and thus to create heat? If the fuel all burns up at source, or just after, all you have is hot air blowing over the shield. 479
To cause damage, it would have to be super-heated air.
Also, it depends on the speed of the flame (the speed of the air flow). An intense flame, but without any force behind it, would not reach 20 feet. Any heat source hot enough to effect the target would be too close to the wizard. Think about standing over a fire, vs standing 20 feet away. So you need a flame with some velocity behind it.
Wood is actually much better at withstanding burning than is metal. A metal beam will deform far earlier than will a wooded beam. Wood doesn't melt, and it doesn't deform and bend under high heat. It maintains its structural integrity until complete failure. It snaps.
And leather is actually very good at protecting against flames. Blacksmiths and welders use leather gloves and aprons for a reason. I have never seen a blacksmith using a metal apron or a chain metal glove.
So, a white or silver curved wooden shield protected by leather (on the inside?) would withstand a prolonged attack.
Unless the wizard could maintain the flame for some time, directed at the target (would the target, a person, really just stand there?) for probably a minute or so, I would suggest that the target could get to the wizard faster than the shield would ignite. And even if the shield DID ignite, it would continue to offer protection until completely burned through.
But change the wizard's weapon to a ball of flame, or a very wide flame (wider than the shield), that will encompass the entire target, and you have a completely different story. The flame does not have to burn through the shield, it (and the heat) goes around it. It's the hot air that wouldn't be stopped. Drop the target into a furnace, and even a ceramic heat shield and perfect insulator will be immaterial, unless the target is COMPLETELY surrounded by the shield.
Really, the best protection against a fire-breathing dragon would be a wooden shield, not metal armor. That's why they make saunas out of wood, not metal.
Edit
My background team has reminded me that there is one other danger besides radiated heat, air temperature, and fuel load.
Traditionally, sulfur and brimstone have been referred to in fantasy worlds.
These burning substances add toxic fumes and carbon monoxide poisoning to the mix. No shield (unless equipped with a respirator) will protect a warrior from products of combustion, particularly sulfur dioxide. If the wizard uses a sulfur based fuel, all survivability analysis and bets are off.
It's not usually the fire that kills, it is the toxic fumes. 160