hi . m about to install 2 by 10kw motors using star delta starter. flc of each motor is 25A. what should be the size of the cable.

I would need to know how long cables are to be and of what type?

There are a lot of ways to determine the cable size since different type of cable may have different ampere rating. However, for minimum requirement, you may use the following formula:

For DC and single phase motor
S= (2.L.I. pf)/(C.VU) or
S = (2.L.P)/(C.VU.U)

For Three Phase motor:
S=(1.73.L.I.pf)/(C.VU) or
S = (L.P)/(C.VU.U)

where
S = cable size in mm sq
L = cable length in meter
I = Rated ampere
P = power of motor in watt
CU = Voltage drop allowable( such as 2.5% of usage voltage)..depend on country regulation
U = Voltage
pf = power factor
C = conductivity of cable if Cu = 56, Al = 35, Fe = 8.5

To get the final result, S shall be divided with the temperature surrounding factor:

20 Deg C: 1.4
25 Deg C : 1.3
30 Deg C : 1.22
35 Deg C : 1.13
40 Deg. C : 1.00
45 Deg. C : 0.87

For example, S = 3.5 with surrounding temperature around 45 Deg. C, then the final result shall be

(3.5)/(0.87). From here, you will select the cable size based on the standard manufacturer available like 1.5, 2.5, 4.0 and etc. For safety, I would suggest you choose one step higher then the final result.