I'm not really a car enthusiast myself (start getting to become one each day tho) hence would hit a wall of knowledge sometimes even with trying.

I've got this 08 350z from my brother and wonder what Tyre size would fit better on the current setting.

Current:
front 18s rays forged with 245/40
rear 19s rays forged with 275/35

Sometimes (really occasionally it would stuck in TCS and slip light turned on issue) and take several attempts/ignition or just wait to get it solved. I don't really sure myself how it was solved either they will just be gone after a while... And also don't really know what caused the problem,sometimes it cccurs when I really hit bumpy route or drove hard but there are many situations that drive normally would make the lights on as well.

I will get it fixed by z specialist anyway, but now I really need to get Tyres replaced anyway.. so if this could potentially caused by Tyres setting I might be able to eliminate that factor first!

 

Those tire sizes are fine and likely not causing the intermittent TCS/Slip. If the tire diameter delta was off - that is, outside of the 3% +/- 0.25% range F-R - the TCS/Slip fault would light up every time you moved the car.

Most likely issue: Check or have someone check your diagnostic codes. Nissans have the odd happenstance of lighting up the TCS/Slip lights if a cam or crankshaft sensor has or is going bad. DTC P034x will show up for camshaft sensors (where x indicates which sensor) OR P0335 indicating a crank sensor malfunction. If one or the other shows up bad, best to replace all three with OEM or Hitachi (the maker of the OEM sensors.) Nissans really don't care much for aftermarket sensors.

There are other possibilities as well but this is a fairly common one.

In the absence of fault codes, there is the rare possibility that one end of the car's tires are worn significantly more on the other end causing an improper delta F-R. But start with a code read.

 

It sounds like you either have the OEM Nismo or the Track V2 wheels in a double-staggered setup. Either way, the specs are the same with 18" x 9" +30 in the front and 19" x 10" +30 in the rear.

OEM sizes are 245/40/18 and 265/35/19.

Since you're running the stock size in the front, that should not cause any issues with VDC/TCS. Running a 275/35 tire in the rear is only a 1% difference and changes the overall diameter by about 7mm, Usually, if you stay within 3%, you won't have any issues.

If the TCS and SLIP lights are coming on periodically, this likely has to do with a faulty cam or crank sensor. They're consumable items. They will sometimes start to fail without causing the SES light to come on. Try reading the codes to see if there is a stored code or maybe a "soft fault."

 

What size are your current tires ?