I have a Legacy LA 290 amp, (400w), and I will be picking up a Mono amp, (1200-2000w), just wondering Guage of wireing I should use on my + from the battery. I will also be using a Distrubition Block because of the 2 amps.
Also, what guage should I ude for ground ? I was told use same Guage as +.

Lastly, how do people connect such large wires, 4g,etc, to their amps, while the slot that you screw the wire into is so small ? Is there some type of adaptor ?

I'd go 0/1 (or 2 4AWG) runs to the distro block, then break it off into 1 8 gauge wire for the small amp, and 1 4 gauge wire for the big monoblock. Same thing with the ground.

http://www.bcae1.com/images/swfs/systemdesignassistant.swf

how to fuse and wire size...

__________________
Car Sound system:
HU: **New** Ural CDD
Speaker: CDT ES-620, crossover: ES-200US, Tweeter: DRT-26 (silk) et TW-24 (silk soft)
Subwoofers: 2 x CDT ES-SQ-1200
Ampli sub: Elemental Design NIne.1 (1200W rms @ 1 ohm)
Ampli Speaker: Elemental Design NIne.2x ( 200 W rms @ 4 ohm )
RCA: Monster Cable 402XLN
Insonorisation: Portes avants et coffre arrière au B-Quiet Extreme
Power: Batterie Otpima Yellow top D35
Wiring: 0 Ga wire

Everything being installed with love an passion and a old lovely Toyota Cressida 89...

Ok thanks.
And for the last question.

how do people connect such large wires, 4g,etc, to their amps, while the slot that you screw the wire into is so small ? Is there some type of adaptor ?

u can use U connector but usually it fits in most amps...

__________________
Car Sound system:
HU: **New** Ural CDD
Speaker: CDT ES-620, crossover: ES-200US, Tweeter: DRT-26 (silk) et TW-24 (silk soft)
Subwoofers: 2 x CDT ES-SQ-1200
Ampli sub: Elemental Design NIne.1 (1200W rms @ 1 ohm)
Ampli Speaker: Elemental Design NIne.2x ( 200 W rms @ 4 ohm )
RCA: Monster Cable 402XLN
Insonorisation: Portes avants et coffre arrière au B-Quiet Extreme
Power: Batterie Otpima Yellow top D35
Wiring: 0 Ga wire

Everything being installed with love an passion and a old lovely Toyota Cressida 89...

Your monoblock should come ready to accept 4AWG, if not 0/1.

Your lil' amp only needs a run of 8 or 10AWG.