I love the name [name_f]Tessa[/name_f], however I feel like it’s too short and nick-name like. I’d love to have a longer name that could be fallen back on if need be, or in more formal settings. I had an idea to use this, considering I also adore the name [name_f]Cassandra[/name_f]. Opinions?
It seems a bit too “smoosh-y” for my taste (think Brangelina or a similar thing). I think [name_f]Tessa[/name_f] is fine on its own or it could be a more modern nickname for [name_f]Theresa[/name_f], possibly 
It does look/sound like a smoosh name to me. While I don’t love it it’s certainly not unusable. I think the more important question is do you love it? I feel like you should love the given name at least as much as the nickname you use. And this isn’t a name I would strongly consider unless I totally adored it.
To be honest, I’m not a huge fan of Tessandra: It seems like a bit of a “smush” name, and I’m not sure I like the parts together. However, I adore [name_f]Tessa[/name_f], and I would definitely say that it’s incredibly strong and professional on its own. I like the suggestion of [name_f]Theresa[/name_f] nicknamed [name_f]Tessa[/name_f], and there’s also [name_f]Teresita[/name_f], [name_f]Natasha[/name_f], and [name_f]Quintessa[/name_f], for a range of styles.
I think that [name_f]Tessa[/name_f] is perfectly fine on her own myself, and think it would actually sound a lot more professional in a formal setting than Tessandra would. Tessandra looks and sounds like a created-smoosh name to me - unnecessarily complex, while [name_f]Tessa[/name_f] is very clean, straight-forward and sweet.
I mean (and as [name_f]Brittany[/name_f] above me similarly said) if you do adore Tessandra as much as [name_f]Tessa[/name_f], then by all means, go for it, however, if you’re just not feeling Tessandra as much and are worried about potential formality primarily, it’d be a pity to downgrade your beloved [name_f]Tessa[/name_f] to ‘just’ a nickname, you know?
If really concerned about the lack of formality, perhaps you could give [name_f]Tessa[/name_f] a longer, less abbreviated middle name instead, to, as you say, fall back on should she need or want to some day? [name_f]Tessa[/name_f] pairs beautifully with a lot of longer (or shorter) classic types - are there any of those types of names you like, that you think work with [name_f]Tessa[/name_f] as first, that you feel are more formal?
I don’t like Tessandra, sorry. I agree it sounds like a rather clumsy smoosh, losing the sleek, mature style of both [name_f]Tessa[/name_f] and [name_f]Cassandra[/name_f].
I second the suggestion of [name_f]Theresa[/name_f] as a longer form if you think [name_f]Tessa[/name_f] needs one (I personally love it on its own), as that’s the traditional long form. There’s also [name_f]Thessaly[/name_f], [name_f]Thomasin/name_f/[name_f]Thomasina[/name_f], [name_f]Thalassa[/name_f], [name_f]Anastasia[/name_f] and I’ve even seen Tessamine on here before, which I think is pretty and feels much more organic than Tessandra as smoosh names go.
I agree with PPs that it sounds a little smooshy. My daughter’s name is [name_f]Tessa[/name_f], and we love it I do sometimes wish we gave her the full name [name_f]Theresa[/name_f]- [name_f]Tessa[/name_f] is a diminutive. It would give her more options as an adult, although I do think [name_f]Tessa[/name_f] stands on its own suits grown ladies too
Not a fan.
As far as “smush” names go, this one is very nice, but I’d personally stick to something more familiar. I love @katinka’s suggestion of [name_f]Thessaly[/name_f], and [name_f]Therese[/name_f] “[name_f]Tess[/name_f]” is one of my favorite names.
[name_f]Tessa[/name_f] is a gorgeous name alone. Tessandra would cause your child difficulty for their entire life. It is the equivalent of Renesme but worse.