I'm using a 42t sugino messenger crank. It's entirely flat where I live, but going over bridges I can pick up some speed, but never more than like 30-32mph and I can't sustain it cos my legs are just flapping all about... Bigger chain ring = harder from a stop but higher top end right? And lower cadence? I don't really know... Guidance?

yeah, you got it right.
are you riding a langster, per chance?

Yeah, a Langster... Do you think I should go with a 48? Will I be able to push much further into the +30 mph realm riding on 48? Will I need a new chain? Will I have to get an entirely new crankset or can I keep the sugino arms and the BB? I just have so many questions...

I have langster, and i jumped to a 46. Noticeable difference in the cadence, i dont find myself spinning too fast on any sized hill. You'll definitely be able to go faster, but your top depends on you of course. The initial push is a bit harder, but you'll get used to it pretty quick. You can keep the crank/bb but you may have to get a new chain. With my 46 and the stock chain there wasnt enough clearance (the rear tire would rub on the brake bridge). So if you plan on getting a bigger chainring, a new chain would be the only other thing you would need.

hope this helped

Stay with the same chainring, or whatever chainring works for most of your ride, and learn to spin.

Last year I geared way down so I could keep riding during chemotherapy, and to my suprise my spin improved dramatically, so that I get around town now just as fast in a low gear as I did in a high gear.

Give it time and eventually your legs will comfortably go whizzzzzzz.

And your spin out cadence is?

Lets see whether the OP is spinning near max to begin with before looking to a gearing solution.

Are you riding with 42t also?

I was just gonna get a sugino, cos I thought it'd be a guaranteed fit.... it seems as though they're different sizes? does anyone know what the sugino 42t messenger crank size is?

https://images.google.com/images?q=money

your sig cracks me up

I'm not big on frills...

What RPM are you spinning at? This guys seems to want to ride in the 35 MPH range, and I dont know too many people that can ride that with 70 G.I.

It should be 130mm BCD or 144 mm BCD. Once you find that out you can decide which chain rings will fit.

How can i find this? Do I take off the ring and measure the space within it?

thanks

https://sheldonbrown.com/gloss_bo-z.html#bcd

you answered your own question (given the same rear cog size)

or google search messenger RDs and look at the BCD size

I've got it now, thank you all.