Awesome!

Lots of people on the forum have very dynamic ponds that are as small as 0.5 acres. Of course, they manage those pretty intensively.

Many more options at 0.75 acres, even more at 1 acre.

Do you have a backhoe or mini-excavator available for use? A silty-loam soil with as little as a 20% clay content can frequently be compacted to form a seal for ponds up to 10 feet deep. (Not guaranteed though at lower 20-25% clay percentages. There are other variables such as the plasticity of the clay, etc.)

If you could dig two test pits and take some samples, you might be able to send them off for particle size analysis to determine your clay content. (There is usually some clay in a silty-loam soil. Also, the clay content might increase with depth, that would give you better material to finish off your pond bottom.) If you have decent material, then your pond size almost becomes a function of your available room and your budget. Since your pond will be excavated, you will need to plan how to deal with the spoils (removed material). You can easily do the math for a long, narrow rectangle and make an estimate. Then add 20% more volume. Excavated material will always "fluff" to a larger volume.

Will you have a good view of the pond from your house or cabin? If so, I personally think some small hills beyond and to the sides of the pond look very nice - especially on otherwise flat ground. That might be a good way to get rid of your spoils. You can then enhance your new terrain with whatever landscaping vegetation you prefer - just don't use anything that will drop leaves that will eventually blow into your pond!

It does look like there is some filamentous algae growing in your pond. That is probably the reason for the steep shoreline. The more shallow the slope, the more photosynthesizing plants grow from the bottom - including the filamentous algae.

If you do expand the pond, then I would try to keep the steep shoreline in most places, except for a swimming beach, spawning beds, etc.

Any work you do must stay within the regulations. When the spring water exits your pond, does it go into a defined waterway? Is there a dashed blue line on the topographic map at that point? If not, then you might have room to expand your pond into that area. The limit of your expansion will probably be the edge of the seasonal "wetlands" on your property. You also cannot put spoils in the wetlands. (Sometimes seasonal wetlands are NOT technically wetlands in the regulations. If the plants and soil type and structures are deemed "terrestrial", then it is not a wetland.)

Once you have your rough plan, then confirm you are compliant with your county, state, and sometimes federal regulations. This is usually a good time to consult with the NRCS agent for your county. Most of the time you get a really good and helpful person. However, I have heard of some people that get a "No" man. In that case, probably best to keep them off of your property if you have some other expert guidance that keeps you within the regulations. Do you know any of your neighbors well, especially the ones with bigger spreads? They might be good info sources about working with your NRCS office.

I agree with you on the broken concrete for unsightly looks. If you do bring in heavy equipment for a pond excavation that is the best time to drag out the concrete blocks, dig out the Russian olive trees that need to be removed, etc. If you are going to rock your pond edges, buy full dump truck loads and finish off that job before releasing the relevant equipment.

If your spring provides "excess" water all year, then you should be able to maintain a stable water level set by the elevation of your outlet pipe. In that case, you could put down heavy landscape cloth and rock the edges from a little bit above the water line to a little below the projected water line. That will help somewhat with your maintenance duties to mow to the edge of the pond and the problems of rooted plants in the very shallow water. However, there is a trade off - the more rooted plants you have in the pond, the more nutrients they utilize and make unavailable to the filamentous algae! (You might want to read a few threads on beneficial pond plants.)

P.S. Based on your description, there is a chance that you actually have a "groundwater pond" rather than a sealed pond fed by a spring. Seeing a distinct flow out of your pond that is roughly the same volume as the water flow into your pond would be the best indicator of a "sealed pond". If you do actually have a groundwater pond, several of the things I said above are slightly "wrong". However, you still have the option of expanding a groundwater pond.

Hope those additional ideas I threw out help you with your further planning!