Please help me size/select a dimmable power supply for an exterior LED flood light application .

Each LED Module is rated for 10 Watts at 12 VAC (Power Factor = 0.8)
There are 5 Fixtures being controlled together

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Your Load is calculated as follows:
5 Fixtures x 10Watts each/Power factor of 0.8 = 63W
If you're not sure this will be the real load (just using info from a cut sheet), we recommend adding 20% safety margin in case the actual load is greater than expected.
63W x 1.20 = 75W

Now that you know the load, you can select the right unit for the application

Powering outdoor low voltage lighting you can select an above grade unit (see examples below) or choose from the direct burial units instead. 

(Above Grade) Mini LS Model LS96-120-12-1x8 (96W maximum load; 12 VAC x 8Amps)

This Mini-LS unit has a single boost tap.  You can utilize our online voltage drop calculator to determine If voltage drop will be a concern or not for what is being powered.

The standard LS unit provides multiple output voltages that can be utilized simultaneously.  Use this unit if you want additional capability to boost voltage to the load.
(Above Grade) LS Model LS120-120-12-1x10 ( 10Amp max load)

Some Final Notes to Consider
LEDs are driven by constant DC current/amperage, so you know this unit has integral drive components inside it to accept AC voltage and convert to DC current at the LED. 

The Power Factor being less than 1.0 is also an indicator that there are electronic components involved.  While most people use "wattage" to size transformers and power supplies, in reality these are sized using VA (Volts x Amps) and power factor plays a role when sizing to load.

Dimmers used with magnetic transformers need to be zero-offset dimmers (also known as Low Voltage Magnetic Dimmers)

Semper Fi's magnetic transformers will function very nicely with 1 Watt or Full Load. When electronics are introduced (dimmers, drivers, etc), be aware of minimum load requirements.