There's one more thing than radius: Game Maker default parameters image_xscale and image_yscale.
e.g. image_xscale*=2; image_yscale*=2;
(Radius 500 is huge - like a width of screen - you mean 50? Big Pokey has radius=32...)
To apply it to bits you have to put the code to cutom event, because at the beginning there's no bit:
m_emitter.bit_number=1; custom_event="with(m_bit) if(radius<50) {radius=50; image_xscale*=2; image_yscale*=2;}"
You have to experiment what factor for scales is good.

Because I need to enlarge the bit monster like double times the size of a normal bit ball.

I guess radius 28 or 30 would be enough. In general radius is important for m_bomb_d and m_pokey, because they are internaly squares (and radius is used for calculating rotation speed). m_bit is circle, so you can put even too big radius and it won't lead to problems, but scale params are important. m_bit has default image_xscale=0.6;image_yscale=0.6;radius=15;.

Never mind, I scrapped the idea, but thanks for the coding example!

Another thing, how do I separate sounds between objects. For example, I want the Fuse Bonus to have the normal sizzle sound and a Bomb Monster to have a nuke sound.

It cannot be done in any easy way. Ignition sounds are played automaticaly. You can only play additional sounds with "custom event":
m_bomb_s.free=1; custom_event="with(m_bomb_s) if (ignite && free) {sound_play(sd_water); sound_stop(sd_ignite); free=0;}"
If you change sd_water sound (or other not used sound, like sd_bit_death) into nuke sound it should work.

Another thing, can two custom events can be used in a level? For example, I want falling blocks and monster having a can together.

It's same problem like "can two ALCs be used in one level". "custom event" is code executed in every step, so you always can add more instructions.

Please give me an example.

To merge fallin bombs code with bit-balloon write:

with(m_emitter) motor=instance_position(x,y,m_balloon); custom_event = "with(m_emitter) locate(id,motor);if (instance_number(m_bomb_d)<10) instance_create(64+random(room_width-128),-32,m_bomb_d);";

You can use water with any codes using:

execute_file(maindir+"levels\water.alc.txt",200,1,$337733); custon_event+="with(glut} move_wrap(1,0,15);";

Interesting... This is more innovative!

Interesting is vertical wrap with water

That would be weird and more of a supernatural world that the world doesn't exists.