I understand your need. I personally recommend checking out the ScanMyPhotos service (link below), as they offer a variety of additional services and customization options for orders. They also provide detailed information about various aspects of their service. Although I've seen they explain the maximum print sizes for scanned slides, I suggest contacting them directly to confirm if they also offer printing services. It's usually better to first have your slides professionally scanned into a high-quality digital format. This way, you'll have the flexibility to print them as you wish or use them for other purposes. If you decide to focus solely on scanning your slides to digital, then iMemories might be the best option for you. Once you have high-quality digital scans of your slides, you can print the photos yourself or use another online service to get them printed as standard glossy prints. If you have any further questions, please feel free to ask.

ScanMyPhotos
iMemories

Ron