• One of the classic grammars for learners of French is Nouvelle grammaire française by André Goosse and Maurice Grevisse (De Boeck, 1995, 396 pages). Size is 23.5 by 15.7 cm, so not "pocket size".
• Bescherelle La grammaire pour tous (Hatier, 2012; 320 pages; 19.6 by 13.9 cm) is a bit more compact. It contains clear explanations and many commented examples. The book is available both in print and in PDF format.
• If you want more than just grammar, you may also consider Bescherelle L'essentiel (Hatier, 2013; 432 pages; 22.7 by 15 cm), which is a bit less compact but which is intended as a reference work for grammar, spelling, conjugations, vocabulary and "expression".
• For a pocket-size book, you might consider Grammaire du français by Delphine Denis and Anne Sancier-Château (Le Livre de Poche, 1997; 541 pages; 17.8 by 10.9 cm), which is apparently very systematic and goes beyond the basics (possibly beyond what you need for level C1).
• The most recent recommendation I received in a course at level C1 was for a book that combined grammar explanations with exercises:
  • Grammaire progressive du français - Niveau intermédiaire by Maïa Grégoire and Odile Thievenaz (CLE International, 2017; there is a separate volume with "corrigés"). Even though the cover says "A2 B1", this book can still be useful for people who can speak and read at level C1 but haven't look at French grammar for a while.
  • The follow-up volume is Grammaire progressive du français - Niveau perfectionnement by Maïa Grégoire and Alina Kostucki, for the levels B2 – C2; also with exercise corrections in a separate book. These books are not compact in the sense that the pages are at A4 size.

There are other reference works that delve deeper into the subtleties of French grammar, but that are aimed at university students and linguists. I have not listed these here.