i am getting a new iMac and a 4 tb hard drive i want to buy a ssd drive for time machine how big should i get

Normally you use a backup drive at least twice the size of the data drive. There is no reason to use a SSD for Time Machine. A normal hard drive is fine. Time Machine is not fast, and a SSD would be a waste. The initial backup may take a while.

Use an external, 7200 RPM, 8 - 10 TB 3.5 inch drive and put it in a USB 3.1 or 3.2 Gen 2 drive enclosure. Ideally, this would be a drive intended for NAS usage (extra durability) but that also boosts the price.

I use this Startech external drive enclosure, which ships with a USB-A cable which would work with a new iMac's USB 3 ports. This is a plug-n-play device with no additional software required. Mine has been in use for four years continuous without any issues using a 2TB Toshiba N300 drive.

Add my vote for using a rotational drive for TM. I have an OWC Mercury Elite Pro USB3 enclosure with a 6GB/sec, 7200RPM Western Digital "Black" drive inside. No TM backup after the first one was slow or intrusive into my workflow. Costly big SSDs would be a waste of money.

To use SSD for TM you would need to make a RAID configuration. by putting several smaller SSD's together. I do not believe you will find a 8TB SSD (double your internal drive). I agree with the others that in your case using a large mechanical drive like perhaps the WD Gold series would provide what you want. I have used both (currently have SSD RAID setup) but I only went this way because I had a several extra SSD's that could use.