There's a specific type of insect that's very large, looks just like a mosquito, but isn't a mosquito at all; it's something known as a Mosquito Hawk. It's large enough ~ if you take full span of the legs into consideration ~ to fit on a quarter, with a little bit of leg leftover. They really are HUGE in appearance, and look rather menacing.
Mosquito Hawks feed on mosquitoes; they eat the mosquitoes that feed on humans and other mammals ~ the ones that require a blood meal. For the record, it's female mosquitoes that need blood meals to nourish their young, so only female mosquitoes "bite".
Mosquito Hawks are supposed to be beneficial, in that they rid the area of harmful mosquitoes, which could carry disease; it's the mosquito that carries West Nile Virus and infects the bird population. I've had several Mosquito Hawks in my house and I kill them ~ it's just too much to ask, in my opinion, to allow those huge things to live in my house!
If what you're asking about looks like a really, really BIG mosquito, it might be a Mosquito Hawk. In that case, they're harmless to humans, and you can ignore them if you choose. They'll eat the mosquitoes in your area that could infect you with harmful diseases like West Nile Virus.