I bought a surface 5 laptop. When I connect to my 49-inch ultrawide monitor I am unable to see the option to select 5120 x 1440 resolution.

Hello Krishna Santosh Vatsavai,

Welcome to Microsoft Community.

I made some queries about the parameters of the Surface Laptop 5:

Troubleshoot connecting Surface to an external display - Microsoft Support

The 5120 x 2880 resolution can be output through the Thunderbolt 4 port without the help of the Surface Dock, so 5120 x 1440 should be supported.

But I notice that you're using an ultra-wide monitor, so the regular resolution built into Windows may not match your monitor. If this type of monitor uses some special resolutions, you may need to install the monitor related drivers, please contact the monitor supplier or after-sales support team to obtain the drivers.

After you install the appropriate driver, you should be able to see the appropriate resolution options.

Another solution is to try to use Intel Graphics Command Center to achieve, through it, you can achieve custom resolution, here is a customer to share the relevant thread, you can refer to: 

Surface Dock and Ultra Wide Monitor 5120 x 1440 Solution - Microsoft Community

Best Regards,

Mitchell | Microsoft Community Support Specialist