I wouldn’t depend on the stored size, as the file might be compressed.
Instead you could calculate the number of parameters and buffers, multiply them with the element size and accumulate these numbers as seen here:
model = models.resnet18()
param_size = 0
for param in model.parameters():
    param_size += param.nelement() * param.element_size()
buffer_size = 0
for buffer in model.buffers():
    buffer_size += buffer.nelement() * buffer.element_size()

size_all_mb = (param_size + buffer_size) / 1024**2
print('model size: {:.3f}MB'.format(size_all_mb))
> model size: 44.629MB