"slightly too long" is correct and usable in written English.
You can use it to express that something is longer than it should be, or more drawn out than necessary. For example, "This explanation is slightly too long; can you make it a bit shorter?".
The reason, Mr. Blodget said, was that the excerpt seemed slightly too long.
The one complaint I have about this book, as with her last, is that many of her essays seem slightly too long.
The Julian calendar, however, made the solar year slightly too long by adding a full quarter of a day annually the solar year actually runs 365.2422 days.
If it did not drag on for slightly too long, "Bioterror" would be a prime example of that all too rare television commodity, the scientifically accurate horror-thriller.
In fact, the preview performance on Monday night ran almost three hours, but to Mr. Reilly's credit it seemed only slightly too long.
Whenever the camera stayed on a character taking slightly too long to sip a drink, you expected throat-clutching death within the hour.
Stephen Campbell Moore catches perfectly the obduracy and awkwardness of Maurice Wilkins, forever tugging at his slightly too-long sleeves.
Collaboration is Ty's default mode, and, perhaps out of habit, he brings too many voices onto the slightly too-long "Beach House 3".
Instead, with his elegant if slightly-too-long sports jacket, voluminous briefcase, packet of sandwiches and air of benign enthusiasm he was a far more approachable figure.
Draw a circle on a whiteboard with a slightly-too-long pen.
For two-thirds of its slightly-too-long run time, this is as good as triple-A action games get these days, and it's absolutely the best-looking video game yet on the PS4.
