Sounds oversized with not enough supply air. Let me guess, the supply registers are a bit loud? Was a Manual J load calculation done to determine sizing and a Manual D performed to design the ductwork? I think not.

Can you describe how the system works? Noise, run times, airflow, etc. etc.

I take it the 20x20= 400-sq.ins., Return Air Filter Rack.

I cool over 850-sf with a HALF-TON window unit in 104-F Heat Index. That is equivalent to 1700-sf per ton of cooling.
I have years of records & witnesses to totally backup my claim.

Why would you need 2-ton for 750-sf?
If it was installed properly, bet it will be short-cycling, which wastes energy big-time.

As Ryan asked, why no manual J?

It is difficult to find one ton central AC equipment, however, depending on where you live, etc., one ton ought to cool that small area. A manual J heatload calc should always be done, along with manual S (sizing), & a manual D (ductwork).

If you want efficient cooling, Airflow is supreme!
Look at the old farm home in the linked page that I cool.
I use "less than a half-ton unit upstairs, usually only in the evening, to cool one bedroom, the hallway/stairwell & bathroom using two fans for air circulation. - udarrell

Here is a link that might be useful: Optimizing Room Air Conditioner Efficiency

Just from glancing at what you have to work with, it seems your a little on the shy side in the ductwork area.

@.08 static pressure, 8" handles 200 cfm
6" handles 95 cfm
4" handles 35 cfm
Add what you have... (2)8"=400,(2)6"=190, 4"=35 and i come up with around 625 cfm. 2 Ton airhandler should move around +/- 800 cfm of air, so you might need another register to help it work better. Like ryanhughes stated above, its probably a bit on the noisy side. IMO

Hope this helps

The system does not really seem to short cycle to bad but you can definitely hear the supply air flow ( it blows hard ). It seems to run for at least 10 minutes during each cycle. I measured the temp differential and it is about 14 to 15 degf. I live in Texas and as you may know, it is hot here. I am sure no calculations were done as I was away on buisness . Since the system is installed what would you recommend ?

Temp split of 14-15* is a bit on the low side. The best thing to do now is to call another contractor (competent and experienced) to come out and evaluate the settings and configuration (i.e. how much airflow is the air handler set to provide--too much could cause the low temp split as can an low refrigerant charge) and make recommendations for improvement. I can tell you that from what you're describe, you need another duct or so added. The runtimes don't sound grossly oversized; how does the system handle humidity? What's your location?

I live in Houston Tx. As far as I can tell the humidity does not seem to be a problem. Is the 14" duct size for the return adequate with or without another supply addition? The room is 45 foot long and the return is at the far end so I am not sure if I may need another return.

As to the runtime of 10 minutes, cycle runtime is predicated on a lot of factors other than Btu/hr tonnage. For how long is the "unit off," then on?

Also, the unit may not be operating at anywhere near its nominal Btu/hr Rating. A nominal Two tons is merely the units POTENTIAL nominal rating, it could be delivering half its "potential Rating."

I have seen somewhat oversized equipment run almost with virtually no cycling, on mild heatload days.

We need at least minimal data to make any ballpark determinations in respect to equipment size.

You can buy a low cost Relative Humidity gauge at most hardware stores. Everyone should have at least one RH gauge.

In TX, do you live in a high or low humidity climate? Summer Design Corpus Christi is around 49% RH, Houston around 46% RH., so I would think you might be in a lighter humidity latent-load area.

We need to know the indoor RH, along with the temperature,and the outdoor temperature.
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List:
Setpoint of the room T-stat

Indoor room Temperature
Indoor relative humidity
Indoor Supply Air/Return Air, actual temp readings:

Outdoor Condenser:
Brand, Model Number, blower CFM, SEER Rating

Outdoor Temp
Condenser discharge air Temperature
Unit cycling times minutes - Ave: time running / time off

Anyone can provide us with this data, & then we might be able to provide any of you with some ballpark rough performance estimates from that data!
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Also, check the discharge air temperature from the outdoor condenser, & if possible, provide the SEER Rating.
- udarrell