I bought this plumbago plant that i wanna use in my perennial border but i dont know where to put it exactly (front or back) and how far from the rest of the plants. I read that it can get up to 10 feet high and wide in its natural surroundings where there's no danger of frost but since my area gets frost, it will never reach that full size mentioned but exactly how big will it get everything before frost comes and kills it to the ground? If anyone has any experience of growing this beautiful speciman, please let me know as far as spacing is concerned.
Plumbago can tolerate a lot of sun and heat so if its in shade, it might not get as big as otherwise. I think that in this area (central TX) they are treated like a "cut and come again" perennial which means they usually freeze to the ground and even if they don't freeze all the way, they will be looking messy so you cut them back to a foot or two. I lost the plumbago that I had but have several other plants in this category including some TX natives. If you have a very mild year with only light to no frosts, then the plant will be much bigger the next year than you are used to.
Exactly how big they will get in your area during an average year will depend on soil, rainfall, etc. If you include your location in a question, people can give you a better answer. Also, another good place to ask is at a locally owned plant nursery.
From the image you supplied, it looks like Plumbago auriculata, which can get up to 4m in height, in theory. Its spread is 1-1.5 metres, but although it can reach up to 4m in height, it will only do that if it is supported, the way you would support a climbing plant, otherwise it's more of a scrambler, growing over itself in a mound, or over other nearby plants.
How well it will do in your climate is literally down to how cold it gets - it will tolerate down to 1deg.C, but prefers temperatures above 5degC - it will be killed by zero degrees or any heavy frost with that sort of temperature. Frost occurs at around 5degC unless the weather is windy, but that will only be a light frost, so if your thermometer outdoors drops below 1degC for a few nights, it may kill it rather than just cause a bit of damage.
If your low temperatures are never below 1degC, then take into account its spread before choosing its spot, which should be sheltered - if you're going to support its growth, then placing it nearer the back seems optimal. Pruning of any dead parts and for reshaping can be done in spring. Further info here, though it's a UK link https://www.rhs.org.uk/plants/13333/Plumbago-auriculata/Details In the UK, this plant is not hardy at all.