Whats the Fan size

There is no answer because you did not provide a model number.  Not all dells have front and back fan slots.

its a dell inspiron 5675 gaming pc

The case fan is 92 mm.  There's no way a 120 mm fan will fit the system case -- the case is too narrow.

can you suggest some liquid coolers since they advertise a liquid cooling  on the gaming pc

I'd suggest contacting Dell Spare Parts for the OEM setup.  If you remove the OEM fans, you'll wind up with boot errors ever time you start the system - the OEM  setup won't produce problems, and will work with the Dell hardware and firmware.

In the US,

1-800-357-3355

If a non-Dell liquid cooler requires you to replace the CPU fan, you might have an issue.

Dell OEM CPU fans have a sensor that's read by BIOS. Don't know If you use a non-Dell liquid cooler if you'll get a "Fan failure - Press F1 to boot" error message every time you boot.  Could you live with that? Certainly something to check out before buying anything..

You can probably order Dell's own cooler for this system by calling Dell Spare Parts at 1-800-357-3355. Have your Service Tag handy...

I just contacted dell and they said they dont sell the liquid cooler on its own I need to buy the whole PC for me to get one, which is bummer, so is it possible for me to get any other liquid cooler. Also the case size fans are 92mm liquid coolers only come in 120mm so how on earth can dell fit in liquid coolers.

So please tell me what I can do is there any external liquid coolers that can fit.

Thanks for the support, do you know any links to coolers that can fit in my case I really wanna overclock my ryzen 5 1400 but my stock cooler cant handle above 3.8ghz so I need something that can cool it down more even if its air cooling i dont mind.

so please if there are any links that I can check out can you give it, thanks.

In all honesty, this isn't the type of system that needs liquid cooling -- it's a budget- to mid-range mainstream system.  If you really must do liquid cooling, there are plenty of them out there that place only the heatsink and pipes inside the case, with the radiator and fans sitting outside.

It's essentially just for show anyway -- the system is perfectly adequately cooled with the stock heatsink -fan (and Ryzen CPUs are generally quite economical with power -- the one I'm typing on runs very cool).

You're not going to get that level of overclocking out of a 1400.  CPUs that can take that would have been shipped as 1600 or 1600X parts.

You can fit a system with an external radiator.

Example:

www.eteknix.com/.../