I don't see a model number listed for your current GE microwave and wall oven combination. This would have given me more information about the specific current model that you have. Most combination wall ovens have an installation guide available with specific cabinet cut out dimensions needed for that particular model.
Instead of physically measuring your cabinet cutout dimensions, I recommend researching the installation instructions of your current combination wall oven for the measurements. This document should give you the detailed measurements that you need to replace the current unit. If you do not have the hard copy of the installation instructions, you can check for the document on our Manage My Home website. Follow these instructions to check for your installation guide:
On the home page, hold your mouse over the square labeled "HOME" in the upper left of the page and choose the option "Products and manuals" from the drop-down menu that appears.
Then enter as much information as possible in the search options.
NOTE: You can enter the following information to view the installation guide for a current GE combination microwave/wall oven model --
JKP90CM1CC. To access manuals for
current model, enter the same category, product and brand. Enter your own model number from the sticker on your oven.
If nothing is found, check your product information for errors such as misspellings and be sure you have used only numbers and letters in the model number (no spaces, periods, etc.).
If the manual can’t be found there, please call the Manage My Home Member Services Team at 1-800-803-6775, between 8 a.m. and 8 p.m. Central Time. They will be more than happy to help you track down the manual.
Once you have the dimensions that were needed to install your current combination wall oven, you can use these measurements to shop for a replacement that will fit your current cabinet opening. To physically measure the dimensions, you can refer to pages 4 and 5 of the installation guide for the example that I provided above (JKP90CM1CC). These pages show detailed drawings with required dimensions for this particular model. Key dimensions that are needed for installation considerations include: cabinet width (usually 27" or 30"), recommended height from the floor, minimum cabinet opening depth, minimum/maximum cabinet opening width, minimum/maximum cabinet opening height.
If you need more assistance with your particular model, resubmit your question with the model number.