How to specify the allocated dim of memref

For example, the memref contains 7x3xf32 of valid data:
memref<7x3xf32>

However, due to hardware requirement, the actual allocated dim of the buffer should at least be
memref<8x4xf32>,
or an even larger buffer, etc, due to other requirements.
memref<16x8xf32>

But I do not find a way to specify two dims in memref.
How to specify memref so I can have perfect knownledge of the dim of the data in the buffer, and the allocated dim of the buffer?
Should I use affine_map or an operator similar to memref.assume_alignment to achieve this?
Or create a new type based on memref?

This looks like something that can be specified with strided memrefs: the shape of the memref remains 7x3, but the strides can be specified as 8x4 or more.