How to choose the antenna aperture size

Effective Isotropic Radiated Power is the apparent power transmitted towards the receiver, if it is assumed that the signal is radiated equally in all directions, such as a spherical wave emanating from a point source; in other words, the arithmetic product of the power supplied to an antenna and its gain.

EIRP = G * P = 10^(g/10) * P [W]

G: Antenna gain coefficient

g: Antenna gain [dBi]

P: Power [W]

Following table contains values for the Ku band. The values shown in the table assume having an Universal LNB：http://www.satlex.de/zh/dictionary-term_Universal+LNB.html with noise figure：http://www.satlex.de/zh/dictionary-term_noise+figure.html of 0.7 dB:

EIRP Field Internsity	Antenna diameter	EIRP Field Internsity	Antenna diameter
64 dBw	22 cm	47 dBw	75 cm
63 dBw	24 cm	46 dBw	80 cm
62 dBw	26 cm	45 dBw	90 cm
61 dBw	28 cm	44 dBw	90 cm
60 dBw	30 cm	43 dBw	100 cm
59 dBw	32 cm	42 dBw	110 cm
58 dBw	34 cm	41 dBw	120 cm
57 dBw	36 cm	40 dBw	120 cm
56 dBw	38 cm	39 dBw	135 cm
55 dBw	40 cm	38 dBw	150 cm
54 dBw	45 cm	37 dBw	180 cm
53 dBw	50 cm	36 dBw	240 cm
52 dBw	50 cm	35 dBw	300 cm
51 dBw	55 cm	34 dBw	355 cm
50 dBw	60 cm	33 dBw	400 cm
49 dBw	60 cm	32 dBw	450 cm
48 dBw	60 cm

Following table contains values for the C band. The values shown in the table assume having an LNB with noise figure of 20°K:

EIRP Field Internsity	Antenna diameter	EIRP Field Internsity	Antenna diameter
42.0 dBw	110 cm	34.0dBw	270 cm
41.5 dBw	120 cm	33.5 dBw	280 cm
41.0 dBw	130 cm	33.0 dBw	290 cm
40.5 dBw	140 cm	32.5 dBw	300 cm
40.0 dBw	150 cm	32.0 dBw	310 cm
39.5 dBw	160 cm	31.5 dBw	320 cm
39.0 dBw	170 cm	31.0 dBw	330 cm
38.5 dBw	180 cm	30.5 dBw	340 cm
38.0 dBw	190 cm	30.0 dBw	350 cm
37.5 dBw	200 cm	29.5 dBw	360 cm
37.0 dBw	210 cm	29.0 dBw	370 cm
36.5 dBw	220 cm	28.5 dBw	380 cm
36.0 dBw	230 cm	28.0 dBw	390 cm
35.5 dBw	240 cm	27.5 dBw	400 cm
35.0 dBw	250 cm	27.0 dBw	410 cm
34.5 dBw	260 cm	26.5 dBw	420 cm