Central air conditioner electrical disconnect amperage
In the US, it's required that manufacturers include the following items on condensing unit's nameplate (NEC 440.4(B)):

Maker's name
Rating in volts
Frequency
Number of phases
Minimum supply circuit conductor ampacity
Maximum rating of the branch-circuit short-circuit and ground-fault protective device

You'll simply use the minimum supply circuit conductor ampacity value from the name plate, and NEC 2011 Table 310.15(B)(16) through Table 310.15(B)(19) to determine the proper conductor size.
As for the branch-circuit short-circuit ground-fault protection, you'll have to select one less than or equal to the maximum rating listed on the nameplate.  However, you'll have to choose a device with a high enough rating to handle the inrush current, so you'll want to choose a device close to this value.

tl;dr
To answer your questions directly...

A 30 ampere breaker is the wrong one, if the nameplate lists 25 as the maximum rating of the branch-circuit short-circuit and ground-fault protective device.

For 25 amperes, you're likely correct to use 10 AWG cable.

Usually the disconnect is not fused, and simply functions as a safety disconnect. You'll just have to make sure the disconnect is rated for greater than 25 amperes, and the appropriate voltage.