Hi KL Chary,
Thank you for reaching out to the Microsoft Q&A platform.
Based on your question description we understand that VM size is available but not showing in VM Resize page.
From which existing VM size to what VM Size are you trying to change?
VM sizes require the VM to be deallocated before resizing. If the new size isn’t available, you might need to stop (deallocate) the VM.
Not all VM sizes are available in every Azure region. Ensure that the VM size you want is available in the region where your VM is deployed.
Please check if you have reached your quota limits for the specific VM size or family. Check your subscription’s quota limits.
Please find the below document for your reference:
https://learn.microsoft.com/en-us/azure/virtual-machines/sizes/resize-vm?tabs=portal
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Thank you.