370 to 400 cc High profile round silicone submuscular placement, to small or to big?

Congratulations on your upcoming surgery.
Too big or too small?  That's the ultimate question.
I think there are a few dimensions to explore before answering the question.
What are your expectations?  You could show your surgeon photos from the internet.  He might show you implants and have you try them on in a tight shirt with a shear bra. If he has access to 3D simulation, like Vectra 3D, you might be able to see a simulated outcome. These are several ways for you to see what you might be comfortable with in a certain size.
The physician will then measure you and help you find what implant will fit your chest dimensions. 
Remember bigger is not necessarily better. Larger implants can stretch the skin and weigh more. The results of smaller implants last longer.
So the answer is finding a happy medium, where you like the size, the implants fit your dimensions and you decrease the risks of sagging in the future.
Hope this was helpful 

Thank-you for your question.
Choosing the correct breast implants may seem overwhelming due to the options available for implant material (silicone or saline), placement (under or over the muscle), implant size and shape. Treatment plans are individualized for each patient based on findings during the consultation to meet your goals. 
Selecting the correct implant size and shape is among the most important decisions in a successful breast augmentation procedure. Placement of the identical implant will not produce the same results in all patients due to differences in:
1.Dimensions of the breast 
2.Diameter of the chest 
3.Volume of breast tissue 
4. Height and weight 
5. Breast shape, including breast ptosis and asymmetry
I rely on 3 methods to advise my patients:
1. Dimensional planning 
2. Gel implant sizing 
3. 3-Dimensional Vectra imageing system.
At the time of consultation, measurements are taken allowing for dimensional planning to select the implant which best fits the base diameter of the breast. Gel implant sizing is performed to allow you to visualize augmentation the with the various sized implants. Vectra 3-D imaging has been a very important method in my practice in selecting the correct implant size and shape as it allows you to see what you will look like following augmentation with any of the available implants. I would recommend a consultation with a Board Certified Plastic Surgeon
Best regards, Andrew Lyos MD, FACS